Upgrade mocha and specify minitest dependency

When I checkout the repo and simply run ```rake test``` I get

```bash
~/.../mocha-0.12.7/lib/mocha/integration/mini_test.rb:56:in '<class:TestCase>': No Mocha monkey-patch for MiniTest version (RuntimeError)
```

The problem seems to be, that my versions of mocha and minitest do not play together.

Mocha's dependency is very loose at the moment (>= 0) and there is no explicite dependency on minitest at all.

I suggest to add/change the following dependencies

mocha ~> 0.14.0
minitest ~> 4.7.5 (>=5 introduces some breaking changes)

With these changes, I can simply checkout, bundle and run rake without any warnings/errors.