bump mocha to fix security issue

Author: jackfranklin

package.json

@@ -52,7 +52,7 @@
     "eslint-config-standard": "5.3.1",
     "eslint-plugin-promise": "1.3.2",
     "eslint-plugin-standard": "1.3.2",
-    "mocha": "^2.1.0",
+    "mocha": "^6.1.4",
     "proxyquire": "^1.0.1",
     "sinon": "^1.9.1"
   }