chore(package): update eslint to version 6.4.0

Closes #13

Author: greenkeeper[bot]

package.json

@@ -5,7 +5,7 @@
   },
   "devDependencies": {
     "del": "^3.0.0",
-    "eslint": "^5.0.0",
+    "eslint": "^6.4.0",
     "eslint-plugin-mocha": "^5.0.0",
     "gulp": "^3.9.0",
     "gulp-eslint": "^5.0.0",