Don't autorequire mocha to avoid deprecation warnings

- https://travis-ci.org/rails/rails/jobs/7264822 contains deprecated
  warnings given by Mocha version 0.14
- 632f215da added Mocha 0.14 to Gemfile
- But with the version, require: false was removed
- So Mocha started giving deprecation warnings
- Acc.to Mocha documentation(https://github.com/freerange/mocha#bundler)
  it should not be auto required.