Require PHPUnit for testing

Author: DougSisk

composer.json

@@ -18,7 +18,8 @@
     "require-dev": {
         "orchestra/testbench": "^3.0",
         "mockery/mockery": "^0.9",
-        "satooshi/php-coveralls": "^1.0"
+        "satooshi/php-coveralls": "^1.0",
+        "phpunit/phpunit": "~5.0"
     },
     "autoload": {
         "psr-4": {