Update dependency mocha to v5

Author: renovate-bot

package.json

@@ -14,7 +14,7 @@
   "devDependencies": {
     "chai": "4.2.0",
     "koa": "2.5.3",
-    "mocha": "2.5.3",
+    "mocha": "5.2.0",
     "supertest": "3.2.0"
   }
 }