Update mocha dependency (#298)

Author: laurenzlong

package.json

@@ -33,7 +33,7 @@
   "devDependencies": {
     "@types/chai": "^3.4.32",
     "@types/chai-as-promised": "0.0.28",
-    "@types/mocha": "^2.2.31",
+    "@types/mocha": "^5.2.5",
     "@types/mock-require": "^1.3.3",
     "@types/nock": "^0.54.32",
     "@types/node": "^6.0.38",
@@ -42,7 +42,7 @@
     "chai-as-promised": "^5.2.0",
     "firebase-admin": "~6.0.0",
     "istanbul": "^0.4.2",
-    "mocha": "^2.4.5",
+    "mocha": "^5.2.0",
     "mock-require": "^2.0.1",
     "nock": "^9.0.0",
     "prettier": "^1.13.7",