fix(dependency): support rails versions 4.0.2 and above

Previously the rails dependency was defined with a pessimistic version
constraint (~>). This has been updated to support any version latter
than or equal to 4.0.2
chore(rspec-rails): upgrade to 3.0.2

Previously we were using the rspec-rails 3 beta release. This has now
been upgraded since this release is no longer in beta.

In this release of rspec-rails `mock_model` has been extracted into its
own gem (rspec-activemodel-mocks). Hence this dependency has been added
to project.