Add Ruby 2.0 to Travis CI matrix and update Rails 4 to released version

1) Added Ruby 2.0 to Travis CI matrix
2) Updated the Rails 4 version to the released version
3) Removed the lock files, so we can have different gemsets based on the platform
4) Restricted Mongoid to allowed platforms in Rails 3.2 Gemfile

All test matrix specs run green on Travis CI