Optimistically constrain Mocha to v0.14.x

The API for the latest version of Mocha (v1.0.0) has changed so will not work without a few tweaks (require statements and the use of Mocha::ParameterMatchers at least).

v0.14.0 is the latest version that works (aside from a small deprecation for the require statement) so I've optimistically constrained the version in the gemspec.