Update dependency node-mailjet to v3.3.1

Author: renovate[bot]

appengine/mailjet/package.json

@@ -21,7 +21,7 @@
     "body-parser": "1.18.2",
     "express": "4.16.4",
     "jade": "1.11.0",
-    "node-mailjet": "3.2.1"
+    "node-mailjet": "3.3.1"
   },
   "devDependencies": {
     "@google-cloud/nodejs-repo-tools": "^2.3.0"