Use lower PHPUnit version to fix Failure exception

I experienced sebastianbergmann/phpunit#3564 so I'm now requiring a lower PHPUnit 7.x version for the time being. We can't upgrade to 8.0 yet because we're still supporting PHP 7. A bugfix is already made (sebastianbergmann/phpunit#3566) so we should revert this once that's been tagged.

composer.json

@@ -28,7 +28,7 @@
         "illuminate/routing": "~5.7.0|~5.8.0",
         "illuminate/view": "~5.7.0|~5.8.0",
         "mockery/mockery": "~1.0",
-        "phpunit/phpunit": "~7.0"
+        "phpunit/phpunit": "~7.4.0"
     },
     "autoload": {
         "psr-4": {