Improving tips about installation in the guides [ci skip]
Merge pull request #17992 from andreynering/guides-intallation

Improving tips about installation in the guides [ci skip]
Remove this tip, if the previous statement is true.. ie: The reader sees "Rails
4.2.0", then they should have no problems hacking with Rails. [ci skip]

If they do, then its a bug and it should be reported.