Replace Bourne with RSpec Mocks

We explicitly request RSpec Rails >= 2.14 in order to get the stubbing
and spying features of RSpec Mocks:

https://github.com/rspec/rspec-mocks/commit/e8cae54c46f20128ec16c4355f5898c48a8d2d91#commitcomment-3240149

We don't need to include the `rspec-mocks` gem directly in the `Gemfile`
or directly configure it in `spec/spec_helper.rb` because `rspec-rails`
includes it as a dependency and defaults the `mock_with` configuration
to it.

Benefits:

* Officially supported by RSpec team.
* Compact.
* Good failure messages.