Replace guides.ror.org/v4.2.0 with guides.ror.org

No need to point links to http://guides.rubyonrails.org/v4.2.0/... now
that the Ruby on Rails guides point to 4.2.0.
Merge pull request #18110 from claudiob/point-guide-to-master

Replace guides.ror.org/v4.2.0 with guides.ror.org