chore(package): update eslint to version 6.4.0

Author: greenkeeper[bot]

package.json

@@ -40,7 +40,7 @@
     "@babel/core": "7.6.0",
     "@babel/preset-env": "7.6.0",
     "del": "5.1.0",
-    "eslint": "6.3.0",
+    "eslint": "6.4.0",
     "eslint-config-google": "0.14.0",
     "fs-extra": "^8.0.1",
     "gulp": "4.0.2",