install: Support being run not directly from /root/zulip.
install: Support being run not directly from /root/zulip.

This adds a dependency on the realpath package on trusty; we could try
to remove it if needed, but given that realpath is included in
coreutils on Xenial (and presumably anything else modern), I think
it's reasonable to add it.

Fixes #1797.

dawran6/zulip,souravbadami/zulip,blaze225/zulip,dattatreya303/zulip,Diptanshu8/zulip,zulip/zulip,rht/zulip,vaidap/zulip,PhilSk/zulip,brainwane/zulip,calvinleenyc/zulip,cosmicAsymmetry/zulip,blaze225/zulip,sonali0901/zulip,dhcrzf/zulip,calvinleenyc/zulip,kou/zulip,Juanvulcano/zulip,shubhamdhama/zulip,Jianchun1/zulip,eeshangarg/zulip,peguin40/zulip,amanharitsh123/zulip,dawran6/zulip,Diptanshu8/zulip,samatdav/zulip,zacps/zulip,rht/zulip,timabbott/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,sharmaeklavya2/zulip,zulip/zulip,hackerkid/zulip,souravbadami/zulip,grave-w-grave/zulip,jphilipsen05/zulip,kou/zulip,Juanvulcano/zulip,kou/zulip,vabs22/zulip,ryanbackman/zulip,susansls/zulip,niftynei/zulip,rishig/zulip,eeshangarg/zulip,Jianchun1/zulip,cosmicAsymmetry/zulip,arpith/zulip,susansls/zulip,peguin40/zulip,peguin40/zulip,zacps/zulip,zulip/zulip,vikas-parashar/zulip,jphilipsen05/zulip,arpith/zulip,vikas-parashar/zulip,jainayush975/zulip,amanharitsh123/zulip,tommyip/zulip,Galexrt/zulip,dattatreya303/zulip,mahim97/zulip,amanharitsh123/zulip,punchagan/zulip,blaze225/zulip,christi3k/zulip,sonali0901/zulip,vabs22/zulip,ryanbackman/zulip,eeshangarg/zulip,sharmaeklavya2/zulip,jphilipsen05/zulip,sharmaeklavya2/zulip,brainwane/zulip,amyliu345/zulip,jrowan/zulip,hackerkid/zulip,synicalsyntax/zulip,shubhamdhama/zulip,jrowan/zulip,TigorC/zulip,christi3k/zulip,susansls/zulip,arpith/zulip,isht3/zulip,jphilipsen05/zulip,calvinleenyc/zulip,zulip/zulip,rht/zulip,brockwhittaker/zulip,brainwane/zulip,brockwhittaker/zulip,KingxBanana/zulip,JPJPJPOPOP/zulip,dawran6/zulip,rishig/zulip,KingxBanana/zulip,amanharitsh123/zulip,SmartPeople/zulip,arpith/zulip,souravbadami/zulip,jphilipsen05/zulip,aakash-cr7/zulip,Galexrt/zulip,rht/zulip,zulip/zulip,shubhamdhama/zulip,vaidap/zulip,hackerkid/zulip,vabs22/zulip,andersk/zulip,showell/zulip,synicalsyntax/zulip,dattatreya303/zulip,SmartPeople/zulip,dhcrzf/zulip,amyliu345/zulip,aakash-cr7/zulip,ryanbackman/zulip,zacps/zulip,brockwhittaker/zulip,tommyip/zulip,shubhamdhama/zulip,andersk/zulip,brockwhittaker/zulip,arpith/zulip,zulip/zulip,rishig/zulip,verma-varsha/zulip,PhilSk/zulip,TigorC/zulip,SmartPeople/zulip,JPJPJPOPOP/zulip,verma-varsha/zulip,timabbott/zulip,jrowan/zulip,synicalsyntax/zulip,ryanbackman/zulip,jackrzhang/zulip,tommyip/zulip,tommyip/zulip,rht/zulip,susansls/zulip,kou/zulip,samatdav/zulip,joyhchen/zulip,AZtheAsian/zulip,blaze225/zulip,showell/zulip,timabbott/zulip,KingxBanana/zulip,christi3k/zulip,aakash-cr7/zulip,grave-w-grave/zulip,jrowan/zulip,sonali0901/zulip,niftynei/zulip,j831/zulip,joyhchen/zulip,calvinleenyc/zulip,verma-varsha/zulip,souravbadami/zulip,dhcrzf/zulip,dattatreya303/zulip,Galexrt/zulip,brockwhittaker/zulip,grave-w-grave/zulip,mahim97/zulip,j831/zulip,dhcrzf/zulip,rishig/zulip,Jianchun1/zulip,grave-w-grave/zulip,brainwane/zulip,SmartPeople/zulip,Galexrt/zulip,christi3k/zulip,kou/zulip,AZtheAsian/zulip,jackrzhang/zulip,andersk/zulip,andersk/zulip,verma-varsha/zulip,samatdav/zulip,samatdav/zulip,peguin40/zulip,niftynei/zulip,KingxBanana/zulip,mahim97/zulip,Diptanshu8/zulip,samatdav/zulip,timabbott/zulip,rishig/zulip,Galexrt/zulip,dhcrzf/zulip,mahim97/zulip,KingxBanana/zulip,mahim97/zulip,brainwane/zulip,TigorC/zulip,Galexrt/zulip,jackrzhang/zulip,peguin40/zulip,sharmaeklavya2/zulip,blaze225/zulip,cosmicAsymmetry/zulip,Jianchun1/zulip,peguin40/zulip,vabs22/zulip,rht/zulip,shubhamdhama/zulip,isht3/zulip,amyliu345/zulip,synicalsyntax/zulip,eeshangarg/zulip,jainayush975/zulip,jainayush975/zulip,zacps/zulip,punchagan/zulip,sonali0901/zulip,brockwhittaker/zulip,dhcrzf/zulip,vaidap/zulip,jainayush975/zulip,calvinleenyc/zulip,PhilSk/zulip,jphilipsen05/zulip,jackrzhang/zulip,calvinleenyc/zulip,grave-w-grave/zulip,punchagan/zulip,timabbott/zulip,synicalsyntax/zulip,brainwane/zulip,niftynei/zulip,susansls/zulip,arpith/zulip,AZtheAsian/zulip,vikas-parashar/zulip,showell/zulip,dawran6/zulip,isht3/zulip,cosmicAsymmetry/zulip,dattatreya303/zulip,timabbott/zulip,isht3/zulip,niftynei/zulip,JPJPJPOPOP/zulip,PhilSk/zulip,showell/zulip,amyliu345/zulip,hackerkid/zulip,vabs22/zulip,cosmicAsymmetry/zulip,verma-varsha/zulip,SmartPeople/zulip,sharmaeklavya2/zulip,synicalsyntax/zulip,rishig/zulip,amanharitsh123/zulip,synicalsyntax/zulip,souravbadami/zulip,jrowan/zulip,punchagan/zulip,vaidap/zulip,amyliu345/zulip,aakash-cr7/zulip,isht3/zulip,ryanbackman/zulip,vabs22/zulip,AZtheAsian/zulip,dhcrzf/zulip,ryanbackman/zulip,christi3k/zulip,KingxBanana/zulip,vaidap/zulip,TigorC/zulip,zulip/zulip,andersk/zulip,andersk/zulip,showell/zulip,kou/zulip,isht3/zulip,Juanvulcano/zulip,eeshangarg/zulip,Jianchun1/zulip,j831/zulip,sonali0901/zulip,JPJPJPOPOP/zulip,tommyip/zulip,Diptanshu8/zulip,timabbott/zulip,j831/zulip,tommyip/zulip,amyliu345/zulip,jackrzhang/zulip,Juanvulcano/zulip,sharmaeklavya2/zulip,PhilSk/zulip,blaze225/zulip,mahim97/zulip,punchagan/zulip,jackrzhang/zulip,kou/zulip,showell/zulip,Juanvulcano/zulip,andersk/zulip,jrowan/zulip,susansls/zulip,punchagan/zulip,TigorC/zulip,JPJPJPOPOP/zulip,dawran6/zulip,brainwane/zulip,vikas-parashar/zulip,joyhchen/zulip,souravbadami/zulip,j831/zulip,joyhchen/zulip,Galexrt/zulip,aakash-cr7/zulip,niftynei/zulip,grave-w-grave/zulip,shubhamdhama/zulip,shubhamdhama/zulip,verma-varsha/zulip,AZtheAsian/zulip,Jianchun1/zulip,jainayush975/zulip,dawran6/zulip,jackrzhang/zulip,amanharitsh123/zulip,dattatreya303/zulip,joyhchen/zulip,aakash-cr7/zulip,PhilSk/zulip,Diptanshu8/zulip,jainayush975/zulip,sonali0901/zulip,j831/zulip,SmartPeople/zulip,christi3k/zulip,samatdav/zulip,Juanvulcano/zulip,vaidap/zulip,vikas-parashar/zulip,Diptanshu8/zulip,hackerkid/zulip,JPJPJPOPOP/zulip,AZtheAsian/zulip,joyhchen/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,rishig/zulip,tommyip/zulip,TigorC/zulip,cosmicAsymmetry/zulip,zacps/zulip,zacps/zulip,vikas-parashar/zulip