fix(package): update mailgun-js to version 0.15.0

Author: greenkeeper[bot]

package.json

@@ -16,7 +16,7 @@
     "farmhash": "^2.0.4",
     "include-all": "^4.0.3",
     "jsonwebtoken": "^8.1.0",
-    "mailgun-js": "^0.14.0",
+    "mailgun-js": "^0.15.0",
     "moment": "^2.19.3",
     "password-validator": "^4.0.0",
     "rc": "^1.2.2",