Simplify `git_source` in Gemfile
Simplify `git_source` in Gemfile

Follow up of https://github.com/rails/rails/commit/0b8441bd415c444b8d4afbfc93af79ec7677aa2c

esparta/rails,rafaelfranca/omg-rails,eileencodes/rails,printercu/rails,gfvcastro/rails,schuetzm/rails,illacceptanything/illacceptanything,jeremy/rails,repinel/rails,fabianoleittes/rails,yasslab/railsguides.jp,palkan/rails,gauravtiwari/rails,aditya-kapoor/rails,yhirano55/rails,tjschuck/rails,printercu/rails,yahonda/rails,kmcphillips/rails,kmcphillips/rails,yawboakye/rails,yasslab/railsguides.jp,arunagw/rails,yahonda/rails,shioyama/rails,mechanicles/rails,gauravtiwari/rails,eileencodes/rails,vipulnsward/rails,prathamesh-sonpatki/rails,rails/rails,illacceptanything/illacceptanything,yalab/rails,yalab/rails,aditya-kapoor/rails,deraru/rails,repinel/rails,prathamesh-sonpatki/rails,Stellenticket/rails,lcreid/rails,yasslab/railsguides.jp,yahonda/rails,Stellenticket/rails,starknx/rails,illacceptanything/illacceptanything,rails/rails,tgxworld/rails,illacceptanything/illacceptanything,untidy-hair/rails,illacceptanything/illacceptanything,travisofthenorth/rails,joonyou/rails,illacceptanything/illacceptanything,joonyou/rails,aditya-kapoor/rails,prathamesh-sonpatki/rails,georgeclaghorn/rails,kddeisz/rails,utilum/rails,Vasfed/rails,yawboakye/rails,alecspopa/rails,bogdanvlviv/rails,tgxworld/rails,BlakeWilliams/rails,deraru/rails,yalab/rails,notapatch/rails,felipecvo/rails,ledestin/rails,pvalena/rails,mechanicles/rails,Erol/rails,EmmaB/rails-1,iainbeeston/rails,prathamesh-sonpatki/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,kmayer/rails,georgeclaghorn/rails,mathieujobin/reduced-rails-for-travis,assain/rails,brchristian/rails,kirs/rails-1,notapatch/rails,yahonda/rails,gauravtiwari/rails,untidy-hair/rails,illacceptanything/illacceptanything,Envek/rails,mathieujobin/reduced-rails-for-travis,arunagw/rails,iainbeeston/rails,illacceptanything/illacceptanything,deraru/rails,pvalena/rails,palkan/rails,assain/rails,odedniv/rails,betesh/rails,pvalena/rails,georgeclaghorn/rails,gfvcastro/rails,arunagw/rails,Erol/rails,MSP-Greg/rails,illacceptanything/illacceptanything,travisofthenorth/rails,yhirano55/rails,Erol/rails,Sen-Zhang/rails,notapatch/rails,flanger001/rails,lcreid/rails,schuetzm/rails,MSP-Greg/rails,mohitnatoo/rails,rafaelfranca/omg-rails,ledestin/rails,vipulnsward/rails,flanger001/rails,eileencodes/rails,baerjam/rails,Vasfed/rails,joonyou/rails,tgxworld/rails,vipulnsward/rails,jeremy/rails,mathieujobin/reduced-rails-for-travis,alecspopa/rails,palkan/rails,brchristian/rails,yhirano55/rails,assain/rails,utilum/rails,Stellenticket/rails,iainbeeston/rails,flanger001/rails,bogdanvlviv/rails,lcreid/rails,mohitnatoo/rails,betesh/rails,tjschuck/rails,odedniv/rails,kirs/rails-1,betesh/rails,rails/rails,Envek/rails,lcreid/rails,kirs/rails-1,flanger001/rails,kddeisz/rails,Envek/rails,gfvcastro/rails,tjschuck/rails,starknx/rails,mechanicles/rails,shioyama/rails,yalab/rails,yasslab/railsguides.jp,schuetzm/rails,odedniv/rails,EmmaB/rails-1,yawboakye/rails,kddeisz/rails,jeremy/rails,BlakeWilliams/rails,BlakeWilliams/rails,mechanicles/rails,gcourtemanche/rails,shioyama/rails,yhirano55/rails,kmcphillips/rails,joonyou/rails,Edouard-chin/rails,arunagw/rails,utilum/rails,kamipo/rails,fabianoleittes/rails,georgeclaghorn/rails,brchristian/rails,printercu/rails,aditya-kapoor/rails,schuetzm/rails,Erol/rails,starknx/rails,illacceptanything/illacceptanything,notapatch/rails,Vasfed/rails,felipecvo/rails,rails/rails,printercu/rails,ledestin/rails,fabianoleittes/rails,tgxworld/rails,Edouard-chin/rails,baerjam/rails,gfvcastro/rails,BlakeWilliams/rails,mohitnatoo/rails,kmayer/rails,Sen-Zhang/rails,esparta/rails,esparta/rails,mohitnatoo/rails,untidy-hair/rails,palkan/rails,baerjam/rails,kmayer/rails,pvalena/rails,repinel/rails,betesh/rails,felipecvo/rails,assain/rails,Edouard-chin/rails,illacceptanything/illacceptanything,kddeisz/rails,iainbeeston/rails,bogdanvlviv/rails,tjschuck/rails,illacceptanything/illacceptanything,alecspopa/rails,kmcphillips/rails,deraru/rails,Sen-Zhang/rails,MSP-Greg/rails,rafaelfranca/omg-rails,esparta/rails,gcourtemanche/rails,MSP-Greg/rails,Edouard-chin/rails,illacceptanything/illacceptanything,kamipo/rails,kamipo/rails,gcourtemanche/rails,vipulnsward/rails,yawboakye/rails,jeremy/rails,repinel/rails,fabianoleittes/rails,EmmaB/rails-1,untidy-hair/rails,Envek/rails,Stellenticket/rails,baerjam/rails,bogdanvlviv/rails,travisofthenorth/rails,Vasfed/rails,travisofthenorth/rails,utilum/rails,shioyama/rails,eileencodes/rails