RSpec 3 Upgrade

This PR bumps RSpec to 3.1 as well as:

- Disable RSpec monkey patching
- Update Rake task for running RSpec
This line is causing to specs in `httparty_spec` to fail. With RSpec 3, mocking methods that don't exist on the real object does not seem to be allowed. I would love some advice on what is the best way to resolve this. The only solution I could come up with was to replace the response object with a double