fix(grunt): pre-minifying angular code

The minified js code throws an error when using the directive _hotkey_:
```
 Unknown provider: aProvider <- a <- hotkeyDirective
```

It's due to the minification of the [line 312](https://github.com/chieffancypants/angular-hotkeys/blob/73c0ec1891fa14cd008710e786225dd0201ee7c4/src/hotkeys.js#L312), it's breaking the AngularJS dependency injection.

Here I'm proposing to add ngmin to pre-minify the source code.