Update Travis config to test on ruby 2.1.2 instead of 2.1.1
Update `_prefixes` override to work with Rails 4.2
Update the overall dependencies and test with mongoid 4.0.0 for now on.
Merge pull request #3082 from greysteil/update-travis

Update Travis config to test on ruby 2.1.2 instead of 2.1.1
Update Rails version across all the Gemfiles