mocha warning regarding deprecated usage fixed.

This warning is currently produced when loading mocha-on-bacon:

    *** Mocha deprecation warning: `require 'mocha_standalone'` has been deprecated. Please use `require 'mocha/api' instead.

My commit, 469b4b6, fixes this by simply requiring 'mocha/api' in lieu of 'mocha_standalone.'

I'd suggest a version bump and new release also.