RSpec 3

RSpec 3's syntaxis much clearer. This PR:
- Specifies the RSpec and WebMock versions in the gemspec
- Updates all specs to use the new RSpec syntax
- Configures RSpec to raise an exception if anyone writes old-syntax specs
- Adds `.bundle` to the `.gitignore` file (for all us rbenv users)

Needless to say they build is green.