Revert to an older mailgun PHP library

api/composer.json

@@ -10,17 +10,12 @@
     }
   ],
   "require-dev": {
-    "phpunit/phpunit": "6.3.0"
+    "phpunit/phpunit": "6.3.0",
+    "clue/graph-composer": "^1.0"
   },
   "require": {
     "phpmailer/phpmailer": "^5.2",
-    "mike-koch/mailgun-php": "^2.1",
-    "php-http/guzzle6-adapter": "^1.1",
-    "php-http/message": "^1.5",
-    "php-http/curl-client": "^1.7",
-    "mike-koch/psr7": "1.4.2.1",
-    "mike-koch/promises": "1.3.1.1",
-    "mike-koch/guzzle": "6.3.0.2",
+    "mailgun/mailgun-php": "1.7.2",
     "mike-koch/php-di": "4.4.11",
     "doctrine/annotations": "1.2"
   }