fix(deps): update dependency probot to v12 [security]

Author: renovate[bot]

package.json

@@ -15,7 +15,7 @@
     "express": "^4.16.2",
     "express-sslify": "^1.2.0",
     "helmet": "^3.9.0",
-    "probot": "^4.0.0"
+    "probot": "^12.3.3"
   },
   "devDependencies": {
     "jest": "^21.2.1",