fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:diff:20180305

Author: snyk-bot

package.json

@@ -53,7 +53,7 @@
     "gulp-concat": "^2.6.0",
     "gulp-eslint": "^5.0.0",
     "gulp-header": "^1.8.3",
-    "gulp-mocha": "^5.0.0",
+    "gulp-mocha": "^6.0.0",
     "gulp-mocha-phantomjs": "^0.12.0",
     "gulp-rename": "^1.2.2",
     "gulp-rimraf": "^0.2.0",