Require PHPUnit 6 or 7

Author: patricksamson

composer.json

@@ -20,7 +20,7 @@
 		"illuminate/http": "~5.1"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "~7.0",
+		"phpunit/phpunit": "~6.0 || ~7.0",
         "mockery/mockery": "~1.1",
         "php-coveralls/php-coveralls": "~1.0",
         "orchestra/testbench": "~3.1"