Fix LoadError for mocha/setup

The Resque tests will no longer pass for me with this line. I get:

    /Users/tarcieri/dev/resque/test/test_helper.rb:5:in `require': cannot
    load such file -- mocha/setup (LoadError)

Removing the line fixes the issue.
Merge pull request #888 from tarcieri/fix_mocha

Fix LoadError for mocha/setup