chore(package): update mocha to version 6.1.1

Closes #25

Author: greenkeeper[bot]

package.json

@@ -44,7 +44,7 @@
   "devDependencies": {
     "chai": "^3.5.0",
     "express": "^4.14.0",
-    "mocha": "^3.3.0",
+    "mocha": "^6.1.1",
     "sinon": "^2.1.0",
     "sinon-chai": "^2.9.0",
     "supertest": "^2.0.1"