README: Clean up Mac installation instructions for Virtualbox/Vagrant.
README: Clean up Mac installation instructions for Virtualbox/Vagrant.

* Removes the hardcoding of an old version of Virtualbox (and doesn't
  specify the version to avoid getting stale again over time).
* Flips around the langauge to assume you don't have Vagrant already.
* Makes clear that the first-time installation is a lot slower than
  future runs will be.

Fixes #5.

vabs22/zulip,verma-varsha/zulip,rht/zulip,Diptanshu8/zulip,esander91/zulip,reyha/zulip,krtkmj/zulip,hackerkid/zulip,arpith/zulip,synicalsyntax/zulip,ashwinirudrappa/zulip,kou/zulip,umkay/zulip,mohsenSy/zulip,niftynei/zulip,niftynei/zulip,jphilipsen05/zulip,AZtheAsian/zulip,showell/zulip,alliejones/zulip,peiwei/zulip,gkotian/zulip,ryansnowboarder/zulip,esander91/zulip,vakila/zulip,gkotian/zulip,susansls/zulip,susansls/zulip,umkay/zulip,isht3/zulip,peguin40/zulip,tommyip/zulip,j831/zulip,jainayush975/zulip,brainwane/zulip,andersk/zulip,blaze225/zulip,SmartPeople/zulip,karamcnair/zulip,atomic-labs/zulip,punchagan/zulip,dhcrzf/zulip,niftynei/zulip,karamcnair/zulip,alliejones/zulip,dwrpayne/zulip,ryansnowboarder/zulip,Diptanshu8/zulip,eeshangarg/zulip,ahmadassaf/zulip,atomic-labs/zulip,Galexrt/zulip,j831/zulip,jackrzhang/zulip,vabs22/zulip,dattatreya303/zulip,cosmicAsymmetry/zulip,sharmaeklavya2/zulip,atomic-labs/zulip,cosmicAsymmetry/zulip,punchagan/zulip,dawran6/zulip,jainayush975/zulip,tommyip/zulip,vakila/zulip,SmartPeople/zulip,zulip/zulip,synicalsyntax/zulip,tommyip/zulip,rishig/zulip,vikas-parashar/zulip,vabs22/zulip,aakash-cr7/zulip,christi3k/zulip,TigorC/zulip,Galexrt/zulip,amanharitsh123/zulip,brockwhittaker/zulip,aakash-cr7/zulip,isht3/zulip,SmartPeople/zulip,vaidap/zulip,gkotian/zulip,tommyip/zulip,timabbott/zulip,zacps/zulip,jackrzhang/zulip,sharmaeklavya2/zulip,amyliu345/zulip,Diptanshu8/zulip,mahim97/zulip,dattatreya303/zulip,dwrpayne/zulip,Frouk/zulip,TigorC/zulip,paxapy/zulip,mahim97/zulip,punchagan/zulip,jrowan/zulip,dhcrzf/zulip,calvinleenyc/zulip,karamcnair/zulip,peiwei/zulip,Galexrt/zulip,vakila/zulip,dattatreya303/zulip,AZtheAsian/zulip,ashwinirudrappa/zulip,amyliu345/zulip,eeshangarg/zulip,AZtheAsian/zulip,souravbadami/zulip,atomic-labs/zulip,vikas-parashar/zulip,zacps/zulip,PhilSk/zulip,showell/zulip,jrowan/zulip,ahmadassaf/zulip,rishig/zulip,jackrzhang/zulip,susansls/zulip,zacps/zulip,karamcnair/zulip,vaidap/zulip,atomic-labs/zulip,joyhchen/zulip,grave-w-grave/zulip,KingxBanana/zulip,vabs22/zulip,andersk/zulip,samatdav/zulip,paxapy/zulip,vakila/zulip,jainayush975/zulip,samatdav/zulip,Frouk/zulip,susansls/zulip,dhcrzf/zulip,mahim97/zulip,Frouk/zulip,krtkmj/zulip,reyha/zulip,samatdav/zulip,brockwhittaker/zulip,arpith/zulip,dhcrzf/zulip,ahmadassaf/zulip,calvinleenyc/zulip,andersk/zulip,synicalsyntax/zulip,jackrzhang/zulip,sharmaeklavya2/zulip,Jianchun1/zulip,Galexrt/zulip,Juanvulcano/zulip,gkotian/zulip,SmartPeople/zulip,rht/zulip,aakash-cr7/zulip,Frouk/zulip,PhilSk/zulip,synicalsyntax/zulip,samatdav/zulip,eeshangarg/zulip,vakila/zulip,showell/zulip,rht/zulip,brainwane/zulip,Vallher/zulip,zulip/zulip,brainwane/zulip,AZtheAsian/zulip,esander91/zulip,kou/zulip,showell/zulip,ahmadassaf/zulip,alliejones/zulip,timabbott/zulip,kou/zulip,umkay/zulip,j831/zulip,vaidap/zulip,JPJPJPOPOP/zulip,shubhamdhama/zulip,verma-varsha/zulip,dattatreya303/zulip,rht/zulip,TigorC/zulip,krtkmj/zulip,sonali0901/zulip,rishig/zulip,JPJPJPOPOP/zulip,ryansnowboarder/zulip,Juanvulcano/zulip,joyhchen/zulip,jainayush975/zulip,Galexrt/zulip,arpith/zulip,ryanbackman/zulip,SmartPeople/zulip,dawran6/zulip,gkotian/zulip,souravbadami/zulip,JPJPJPOPOP/zulip,christi3k/zulip,kou/zulip,jphilipsen05/zulip,sonali0901/zulip,andersk/zulip,sup95/zulip,blaze225/zulip,mahim97/zulip,Diptanshu8/zulip,souravbadami/zulip,Jianchun1/zulip,calvinleenyc/zulip,christi3k/zulip,tommyip/zulip,vabs22/zulip,TigorC/zulip,KingxBanana/zulip,cosmicAsymmetry/zulip,sup95/zulip,cosmicAsymmetry/zulip,reyha/zulip,Jianchun1/zulip,amanharitsh123/zulip,KingxBanana/zulip,AZtheAsian/zulip,Frouk/zulip,zulip/zulip,dawran6/zulip,ryanbackman/zulip,amanharitsh123/zulip,vikas-parashar/zulip,gkotian/zulip,jainayush975/zulip,Frouk/zulip,shubhamdhama/zulip,j831/zulip,dwrpayne/zulip,showell/zulip,krtkmj/zulip,esander91/zulip,dwrpayne/zulip,ryanbackman/zulip,sonali0901/zulip,kou/zulip,kou/zulip,jainayush975/zulip,verma-varsha/zulip,arpith/zulip,dwrpayne/zulip,showell/zulip,Juanvulcano/zulip,eeshangarg/zulip,joyhchen/zulip,TigorC/zulip,andersk/zulip,christi3k/zulip,ashwinirudrappa/zulip,reyha/zulip,sup95/zulip,vikas-parashar/zulip,blaze225/zulip,susansls/zulip,grave-w-grave/zulip,krtkmj/zulip,zulip/zulip,christi3k/zulip,vaidap/zulip,zacps/zulip,vaidap/zulip,ryanbackman/zulip,jackrzhang/zulip,vabs22/zulip,Juanvulcano/zulip,brainwane/zulip,ryansnowboarder/zulip,sup95/zulip,Diptanshu8/zulip,KingxBanana/zulip,mohsenSy/zulip,jrowan/zulip,j831/zulip,sharmaeklavya2/zulip,ahmadassaf/zulip,krtkmj/zulip,ryansnowboarder/zulip,niftynei/zulip,umkay/zulip,isht3/zulip,jphilipsen05/zulip,timabbott/zulip,susansls/zulip,arpith/zulip,umkay/zulip,amanharitsh123/zulip,niftynei/zulip,cosmicAsymmetry/zulip,j831/zulip,timabbott/zulip,punchagan/zulip,dhcrzf/zulip,Juanvulcano/zulip,arpith/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,ahmadassaf/zulip,calvinleenyc/zulip,souravbadami/zulip,Vallher/zulip,hackerkid/zulip,esander91/zulip,andersk/zulip,aakash-cr7/zulip,dawran6/zulip,PhilSk/zulip,Vallher/zulip,calvinleenyc/zulip,mohsenSy/zulip,vaidap/zulip,jackrzhang/zulip,esander91/zulip,alliejones/zulip,alliejones/zulip,dattatreya303/zulip,brockwhittaker/zulip,brockwhittaker/zulip,samatdav/zulip,krtkmj/zulip,aakash-cr7/zulip,shubhamdhama/zulip,peiwei/zulip,peguin40/zulip,zacps/zulip,hackerkid/zulip,dhcrzf/zulip,umkay/zulip,dawran6/zulip,amyliu345/zulip,Frouk/zulip,dwrpayne/zulip,sharmaeklavya2/zulip,kou/zulip,blaze225/zulip,souravbadami/zulip,KingxBanana/zulip,shubhamdhama/zulip,PhilSk/zulip,rishig/zulip,shubhamdhama/zulip,zulip/zulip,mohsenSy/zulip,reyha/zulip,peguin40/zulip,synicalsyntax/zulip,synicalsyntax/zulip,Galexrt/zulip,ahmadassaf/zulip,umkay/zulip,niftynei/zulip,dawran6/zulip,isht3/zulip,paxapy/zulip,rht/zulip,ashwinirudrappa/zulip,peiwei/zulip,Jianchun1/zulip,amyliu345/zulip,brainwane/zulip,karamcnair/zulip,atomic-labs/zulip,SmartPeople/zulip,hackerkid/zulip,sonali0901/zulip,grave-w-grave/zulip,amanharitsh123/zulip,reyha/zulip,tommyip/zulip,peiwei/zulip,showell/zulip,sharmaeklavya2/zulip,timabbott/zulip,alliejones/zulip,peiwei/zulip,eeshangarg/zulip,PhilSk/zulip,mohsenSy/zulip,isht3/zulip,Jianchun1/zulip,Vallher/zulip,calvinleenyc/zulip,synicalsyntax/zulip,peiwei/zulip,punchagan/zulip,grave-w-grave/zulip,Vallher/zulip,brainwane/zulip,paxapy/zulip,ryanbackman/zulip,hackerkid/zulip,sup95/zulip,JPJPJPOPOP/zulip,dattatreya303/zulip,mahim97/zulip,verma-varsha/zulip,peguin40/zulip,alliejones/zulip,peguin40/zulip,dhcrzf/zulip,paxapy/zulip,amyliu345/zulip,Diptanshu8/zulip,Juanvulcano/zulip,vikas-parashar/zulip,souravbadami/zulip,jphilipsen05/zulip,aakash-cr7/zulip,jrowan/zulip,zulip/zulip,sup95/zulip,joyhchen/zulip,esander91/zulip,brockwhittaker/zulip,rht/zulip,shubhamdhama/zulip,AZtheAsian/zulip,jackrzhang/zulip,christi3k/zulip,verma-varsha/zulip,JPJPJPOPOP/zulip,hackerkid/zulip,cosmicAsymmetry/zulip,joyhchen/zulip,mohsenSy/zulip,zulip/zulip,PhilSk/zulip,paxapy/zulip,verma-varsha/zulip,jphilipsen05/zulip,sonali0901/zulip,brainwane/zulip,ryanbackman/zulip,rishig/zulip,jphilipsen05/zulip,karamcnair/zulip,isht3/zulip,vikas-parashar/zulip,KingxBanana/zulip,ryansnowboarder/zulip,atomic-labs/zulip,brockwhittaker/zulip,rishig/zulip,amanharitsh123/zulip,zacps/zulip,rishig/zulip,Jianchun1/zulip,grave-w-grave/zulip,mahim97/zulip,timabbott/zulip,eeshangarg/zulip,joyhchen/zulip,vakila/zulip,grave-w-grave/zulip,dwrpayne/zulip,Galexrt/zulip,timabbott/zulip,peguin40/zulip,jrowan/zulip,jrowan/zulip,Vallher/zulip,ryansnowboarder/zulip,gkotian/zulip,vakila/zulip,rht/zulip,tommyip/zulip,JPJPJPOPOP/zulip,ashwinirudrappa/zulip,blaze225/zulip,sonali0901/zulip,ashwinirudrappa/zulip,amyliu345/zulip,samatdav/zulip,andersk/zulip,shubhamdhama/zulip,punchagan/zulip,blaze225/zulip,Vallher/zulip,karamcnair/zulip,ashwinirudrappa/zulip,TigorC/zulip