Add support for Laravel 5.5

[kylestev]

fixes #10

The changes made to ChannelTest and MessageTest work on PHPUnit 4.x as well as 6.0. To test this, I stashed my changes to composer.json and ran $ composer update + $ phpunit.

Here is the updated $ phpunit output for this PR:

PHPUnit 6.3.0 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.1.5
Configuration: laravel-webhook-notifications/phpunit.xml.dist
Error:         No code coverage driver is available

RR......                                                            8 / 8 (100%)

Time: 209 ms, Memory: 10.00MB

There were 2 risky tests:

1) NotificationChannels\Webhook\Test\ChannelTest::it_can_send_a_notification
This test did not perform any assertions

2) NotificationChannels\Webhook\Test\ChannelTest::it_can_send_a_notification_with_2xx_status
This test did not perform any assertions

OK, but incomplete, skipped, or risky tests!
Tests: 8, Assertions: 6, Risky: 2.

[ngugijames]

Please please merge this PR... 🥇

[ngugijames]

thanks!