fix: upgrade mailgun.js from 3.4.0 to 3.7.3 (#16)

Snyk has created this PR to upgrade mailgun.js from 3.4.0 to 3.7.3.

See this package in npm:
https://www.npmjs.com/package/mailgun.js

See this project in Snyk:
https://app.snyk.io/org/davehorton/project/b7e09765-19b2-4aa5-90ba-10432e250041?utm_source=github&utm_medium=referral&page=upgrade-pr

package.json

@@ -31,7 +31,7 @@
     "form-data": "^2.3.3",
     "form-urlencoded": "^6.0.4",
     "jsonwebtoken": "^8.5.1",
-    "mailgun.js": "^3.3.0",
+    "mailgun.js": "^3.7.3",
     "microsoft-cognitiveservices-speech-sdk": "^1.19.0",
     "mysql2": "^2.2.5",
     "passport": "^0.5.0",