Update dependency mysql2 to v3

Author: mend-for-github-com[bot]

package.json

@@ -25,7 +25,7 @@
     "mathjs": "^4.0.0",
     "md5": "^2.2.1",
     "morgan": "^1.9.0",
-    "mysql2": "^1.4.2",
+    "mysql2": "^3.9.4",
     "node-serialize": "0.0.4",
     "passport": "^0.4.0",
     "passport-local": "^1.0.0",