chore(package): update eslint to version 6.4.0

Closes #31

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
   },
   "devDependencies": {
     "@semantic-release/condition-travis": "^6.0.0",
-    "eslint": "^4.18.0",
+    "eslint": "^6.4.0",
     "eslint-config-standard": "^11.0.0",
     "eslint-plugin-import": "^2.11.0",
     "eslint-plugin-node": "^6.0.1",