Fix mocha deprecation warning
Fix mocha deprecation warning

Mocha deprecation warning at /home/travis/build/heroku/hatchet/test/test_helper.rb:10:in `require': Require 'mocha/test_unit', 'mocha/minitest' or 'mocha/api' instead of 'mocha/setup'.
heroku/hatchet,heroku/hatchet,heroku/hatchet