chore(package): update mocha to version 6.2.1

Closes #65

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "eslint": "^4.14.0",
     "eslint-config-tschaub": "^10.0.0",
     "gulp": "^3.9.1",
-    "mocha": "^4.1.0",
+    "mocha": "^6.2.1",
     "mock-fs": "^4.4.2",
     "vinyl": "^2.1.0"
   },