Update to use newest version of nexmo/laravel

[dragonmantank]

(This may be better suited for master. I'm not sure if Laravel considers this a BC-breaking change or not, since the underlying package is being changed.)

This switches this library over to using the nexmo/laravel package, and brings everything up to the 2.x line.

Switching to the nexmo/laravel allows the Nexmo client to be pulled directly from the Service Container, as opposed to being instantiated directly in the Channel Service Provider. nexmo/laravel also ships a Facade, giving the developers the option to use the Service Container or Facades to pull in a valid package if they want to use Nexmo in other places in their application.

We're recommending Laravel switch to use nexmo/laravel as it also gives users a workaround in case they run into GuzzleHTTP collisions in the future (see https://github.com/Nexmo/nexmo-php/issues/5). nexmo/laravel can easily be overridden to use a different HTTPlug-compatible adapter, where directly creating a \Nexmo\Client makes it much harder to override.

Both laravel/nexmo-notification-channel and nexmo/laravel use the same .env options, so this should be a drop-in replacement for users.

[driesvints]

Thanks @dragonmantank. Just released v2.2.0 with these changes.