Make phpunit a dev dependency

api/composer.json

@@ -9,8 +9,10 @@
       "email": "mkoch227@gmail.com"
     }
   ],
+  "require-dev": {
+    "phpunit/phpunit": "5.7.9"
+  },
   "require": {
-    "phpunit/phpunit": "5.7.9",
     "phpmailer/phpmailer": "^5.2",
     "mailgun/mailgun-php": "^2.1",
     "php-http/guzzle6-adapter": "^1.1",