Update to Rails 4 release.

Rails 4 has now been released. Unfortunately, the '>= 4.0.0.rc1'
dependency doesn't work with the release.

Change-Id: I0db87057dfe8f8dd5c9947a21df4b6b7e060d898
Merge pull request #832 from freerunningtechnologies/rails-4-release

Update to Rails 4 release.