run newer phpunit version

Author: lukewaite

composer.json

@@ -20,7 +20,7 @@
     "require-dev": {
         "illuminate/queue": "^5.1|^5.2|^5.3",
         "mockery/mockery": "^0.9.9",
-        "phpunit/phpunit": "4.*"
+        "phpunit/phpunit": "^5.7"
     },
     "autoload": {
         "psr-4": {

tests/PagerDutyChannelTest.php

@@ -5,13 +5,14 @@
 use Mockery;
 use GuzzleHttp\Client;
 use GuzzleHttp\Psr7\Response;
+use PHPUnit\Framework\TestCase;
 use Illuminate\Notifications\Notification;
 use NotificationChannels\PagerDuty\PagerDutyChannel;
 use NotificationChannels\PagerDuty\PagerDutyMessage;
 use NotificationChannels\PagerDuty\Exceptions\ApiError;
 use NotificationChannels\PagerDuty\Exceptions\CouldNotSendNotification;
 
-class ChannelTest extends \PHPUnit_Framework_TestCase
+class ChannelTest extends TestCase
 {
     public function tearDown()
     {

tests/PagerDutyMessageTest.php

@@ -2,9 +2,10 @@
 
 namespace NotificationChannels\PagerDuty\Test;
 
+use PHPUnit\Framework\TestCase;
 use NotificationChannels\PagerDuty\PagerDutyMessage;
 
-class PagerDutyMessageTest extends \PHPUnit_Framework_TestCase
+class PagerDutyMessageTest extends TestCase
 {
     /** @test */
     public function basic_message_has_all_values()