chore(deps): update dependency gulp-mocha to v6 (#29)

Author: renovate[bot]

package.json

@@ -49,7 +49,7 @@
     "gulp-coffee": "2.3.5",
     "gulp-coveralls": "0.1.4",
     "gulp-istanbul": "1.1.3",
-    "gulp-mocha": "5.0.0",
+    "gulp-mocha": "6.0.0",
     "gulp-uglifyjs": "0.6.2",
     "gulp-util": "3.0.8",
     "gulp-verb": "0.3.0",