Some tweaks to the development set-up guide
Some tweaks to the development set-up guide

* Remove the libxml2 and libxslt part ; Nokogiri ships now with these
  libraries bundled.
* Add the missing installation instructions for either Arch Linux,
  FreeBSD or Mac OS X.
* Use the new `pkg install` syntax over `pkg_add -r` for FreeBSD 10+
  users.
* Point to the up-to-date version of PostgreSQL as for the BSD port.
* Make the platform installation instructions order consistent
  throughout the guide.
* Remove useless duplicated links to Homebrew's home.

[ci skip]

brchristian/rails,stefanmb/rails,yawboakye/rails,printercu/rails,arunagw/rails,mtsmfm/railstest,yahonda/rails,tjschuck/rails,kddeisz/rails,starknx/rails,yalab/rails,vipulnsward/rails,samphilipd/rails,coreyward/rails,rails/rails,tijwelch/rails,tgxworld/rails,mechanicles/rails,alecspopa/rails,Erol/rails,kachick/rails,Envek/rails,eileencodes/rails,coreyward/rails,rails/rails,illacceptanything/illacceptanything,elfassy/rails,bogdanvlviv/rails,deraru/rails,maicher/rails,esparta/rails,sealocal/rails,untidy-hair/rails,odedniv/rails,fabianoleittes/rails,kirs/rails-1,Erol/rails,mathieujobin/reduced-rails-for-travis,printercu/rails,assain/rails,jeremy/rails,assain/rails,kmcphillips/rails,bolek/rails,Spin42/rails,notapatch/rails,Vasfed/rails,joonyou/rails,coreyward/rails,mtsmfm/rails,odedniv/rails,iainbeeston/rails,bradleypriest/rails,prathamesh-sonpatki/rails,yhirano55/rails,kmayer/rails,mtsmfm/railstest,illacceptanything/illacceptanything,samphilipd/rails,riseshia/railsguides.kr,Envek/rails,MichaelSp/rails,iainbeeston/rails,gcourtemanche/rails,mtsmfm/railstest,bradleypriest/rails,travisofthenorth/rails,riseshia/railsguides.kr,Erol/rails,mohitnatoo/rails,gauravtiwari/rails,repinel/rails,MSP-Greg/rails,voray/rails,lcreid/rails,kaspth/rails,pvalena/rails,gfvcastro/rails,mtsmfm/rails,richseviora/rails,kaspth/rails,rails/rails,pschambacher/rails,illacceptanything/illacceptanything,Envek/rails,gavingmiller/rails,tijwelch/rails,utilum/rails,BlakeWilliams/rails,georgeclaghorn/rails,ttanimichi/rails,xlymian/rails,kachick/rails,arunagw/rails,yhirano55/rails,bolek/rails,richseviora/rails,felipecvo/rails,kenta-s/rails,voray/rails,brchristian/rails,88rabbit/newRails,untidy-hair/rails,BlakeWilliams/rails,joonyou/rails,arjes/rails,kaspth/rails,riseshia/railsguides.kr,felipecvo/rails,matrinox/rails,tijwelch/rails,baerjam/rails,kmcphillips/rails,rbhitchcock/rails,gcourtemanche/rails,aditya-kapoor/rails,eileencodes/rails,Edouard-chin/rails,samphilipd/rails,travisofthenorth/rails,riseshia/railsguides.kr,vassilevsky/rails,stefanmb/rails,88rabbit/newRails,iainbeeston/rails,palkan/rails,schuetzm/rails,baerjam/rails,repinel/rails,MichaelSp/rails,MSP-Greg/rails,pschambacher/rails,repinel/rails,flanger001/rails,jeremy/rails,sealocal/rails,jeremy/rails,utilum/rails,sergey-alekseev/rails,mohitnatoo/rails,mathieujobin/reduced-rails-for-travis,printercu/rails,MSP-Greg/rails,arunagw/rails,ttanimichi/rails,utilum/rails,yasslab/railsguides.jp,tjschuck/rails,yhirano55/rails,tgxworld/rails,joonyou/rails,88rabbit/newRails,hanystudy/rails,Vasfed/rails,shioyama/rails,BlakeWilliams/rails,yalab/rails,MSP-Greg/rails,untidy-hair/rails,illacceptanything/illacceptanything,rbhitchcock/rails,tjschuck/rails,arunagw/rails,MichaelSp/rails,untidy-hair/rails,stefanmb/rails,Sen-Zhang/rails,betesh/rails,illacceptanything/illacceptanything,georgeclaghorn/rails,starknx/rails,illacceptanything/illacceptanything,prathamesh-sonpatki/rails,Vasfed/rails,illacceptanything/illacceptanything,kddeisz/rails,arjes/rails,kmayer/rails,lucasmazza/rails,rossta/rails,yawboakye/rails,illacceptanything/illacceptanything,rafaelfranca/omg-rails,illacceptanything/illacceptanything,gavingmiller/rails,vassilevsky/rails,yhirano55/rails,kirs/rails-1,esparta/rails,flanger001/rails,kmcphillips/rails,richseviora/rails,mtsmfm/rails,ledestin/rails,aditya-kapoor/rails,kenta-s/rails,alecspopa/rails,amoody2108/TechForJustice,kirs/rails-1,kamipo/rails,illacceptanything/illacceptanything,mechanicles/rails,Edouard-chin/rails,ngpestelos/rails,hanystudy/rails,illacceptanything/illacceptanything,tjschuck/rails,amoody2108/TechForJustice,printercu/rails,palkan/rails,alecspopa/rails,flanger001/rails,gauravtiwari/rails,maicher/rails,lucasmazza/rails,yasslab/railsguides.jp,rafaelfranca/omg-rails,lcreid/rails,voray/rails,schuetzm/rails,gavingmiller/rails,repinel/rails,mohitnatoo/rails,kachick/rails,gcourtemanche/rails,Stellenticket/rails,koic/rails,bolek/rails,sergey-alekseev/rails,esparta/rails,fabianoleittes/rails,rbhitchcock/rails,yahonda/rails,eileencodes/rails,bogdanvlviv/rails,bradleypriest/rails,mechanicles/rails,vipulnsward/rails,ngpestelos/rails,yasslab/railsguides.jp,betesh/rails,kenta-s/rails,ledestin/rails,kamipo/rails,rafaelfranca/omg-rails,pvalena/rails,yalab/rails,travisofthenorth/rails,lcreid/rails,Sen-Zhang/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,gfvcastro/rails,aditya-kapoor/rails,bogdanvlviv/rails,Stellenticket/rails,fabianoleittes/rails,rossta/rails,palkan/rails,deraru/rails,vipulnsward/rails,aditya-kapoor/rails,EmmaB/rails-1,eileencodes/rails,assain/rails,elfassy/rails,BlakeWilliams/rails,rossta/rails,tgxworld/rails,pschambacher/rails,arjes/rails,iainbeeston/rails,Stellenticket/rails,gfvcastro/rails,deraru/rails,jeremy/rails,palkan/rails,yawboakye/rails,betesh/rails,brchristian/rails,elfassy/rails,ledestin/rails,fabianoleittes/rails,Spin42/rails,notapatch/rails,gauravtiwari/rails,xlymian/rails,shioyama/rails,hanystudy/rails,koic/rails,mathieujobin/reduced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