Use explicit dependency injection syntax

> In both cases where the `hotkeys` service is injected, use the explicit
dependency declaration syntax instead of relying on the parameter names.

> This makes the library compatible with variable manglers.

Large projects often concat all their JS into a single file and uglify it. If they mangle variables, angular-hotkeys will break because Angular tries to infer the dependency names from parameters.

**Relevant Angular docs:** [Inline Array Annotation](https://docs.angularjs.org/guide/di#inline-array-annotation)