Adds a version to Gemfile

Seems that latest mocha (0.13.x) is not compatible with rspec 2.8.x This patch
adds a version for mocha to make the tests work out-of-the-box.

More information can bee seen in the following links:

- https://github.com/rspec/rspec-core/issues/727
- https://github.com/freerange/mocha/issues/107
- http://stackoverflow.com/questions/14588424/cannot-load-such-file-mocha-object-loaderror