2.1.x does not work with latest stable ember-mocha release

I've added the `start()` call from `ember-exam` to our `test-helper.js` file (even though the current ember-mocha release doesn't use an explicit `start()` call) which allows our tests to run but I still get this error when attempting to run our test suite and parallelization doesn't occur:

```
Global error: Uncaught Error: Unable to find a suitable test loader. You should ensure that one of `ember-qunit` or `ember-mocha` are added as dependencies.
```

Looking at the [`getTestLoader()` code here](https://github.com/ember-cli/ember-exam/blob/68641d6f8ce5a2208b8a8d5d0336823aa18706ea/addon-test-support/-private/get-test-loader.js#L13-L15) it's checking for the existence of `ember-mocha/test-loader` but `ember-mocha` does not have such an export or file.

ember-exam@2.1.1
ember-mocha@0.14.0