Upgrade to rspec-rails 3.x.x

Should we explicitly specify `~> 3.0.0` here? Reasons:

* If somebody already has rspec-rails 2.x installed, I believe they'll end up with that.
* I think that `infer_spec_type_from_file_location!` method you're using below will cause an error in rspec-rails 2.x.
* rspec changes tend to have a high impact on our applications, so using semantic version tracking seems appropriate.