chore(package): update eslint to version 6.4.0

Closes #18

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "eslint": "^4.1.0"
   },
   "devDependencies": {
-    "eslint": "^4.1.0",
+    "eslint": "^6.4.0",
     "chai": "^4.0.1",
     "mocha": "^3.2.0"
   }