Return after promise rejected.

I'm using this in parse-server, and there was a problem. The promise should have been rejecting, but instead it was rejecting AND resolving. 

Not sure that this is the best fix, happy to discuss further.

Author: ChrisGrant

index.js

@@ -19,6 +19,7 @@ var SimpleMailgunAdapter = mailgunOptions => {
       mailgun.messages().send(data, (err, body) => {
         if (typeof err !== 'undefined') {
           reject(err);
+          return;
         }
         resolve(body);
       });