[2.x] Fix shortcode messaging with the latest version of nexmo-client

[dwightwatson]

With the latest release(s) of nexmo-client (v2.2.0+) they've made some changes internally, which affect how Nexmo\Message\Client uses the Nexmo\Client that is passed into it. This breaks the ability to send shortcodes when you have the latest versions of nexmo-client.

Previously I used the container to create a new instance of Nexmo\Message\Client and Laravel was being smart and giving it an instance of an optional dependency. However, this optional dependency now overrides the custom Nexmo\Client we're setting here.

The easy fix is to just not provide the optional dependency and new up the instance as-is. Alternatively I could register it directly with the service provider so it knows not to pass any dependency but that felt like overkill in this instance.

[driesvints]

It seems https://github.com/Nexmo/nexmo-php-complete doesn't have a 2.0 tag yet. Since nexmo/laravel uses this package we can't merge this until it does.

I've sent in Nexmo/nexmo-laravel#45 so nexmo-laravel might get rid of the unnecessary extra dependency in between.

[driesvints]

Converted this to draft until the other PR has been handled.

[driesvints]

As Chris explained in Nexmo/nexmo-laravel#45 it seems it does already have a 2.0.0 tag. So this PR is gtg for me.