chore(package): update eslint to version 6.4.0

Closes #414

Author: greenkeeper[bot]

package.json

@@ -48,7 +48,7 @@
   },
   "devDependencies": {
     "coveralls": "3.0.0",
-    "eslint": "4.18.1",
+    "eslint": "6.4.0",
     "eslint-plugin-import": "2.10.0",
     "eslint-config-standard": "11.0.0",
     "eslint-plugin-node": "6.0.1",