chore(package): update mocha to version 6.1.0

Closes #31

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "gulp-eslint": "^4.0.2",
     "gulp-istanbul": "^1.1.3",
     "gulp-mocha": "^5.0.0",
-    "mocha": "^5.0.1",
+    "mocha": "^6.1.0",
     "mocha-sinon": "^2.0.0",
     "prettier": "^1.10.2",
     "sinon": "^4.3.0"