Fix LoadError exception and deprecation warnings

* RSpec 2.4.0 causes a LoadError with newer versions of Mocha
  (>= 0.13.0) when it tries to require a missing mocha/object.

* Set up-to-date version specifiers on the rspec and mocha gems.

* Fix deprecation warnings from Mocha saying that `require 'mocha/api'`
  should be used instead of `require 'mocha'`.