chore(package): update eslint to version 6.4.0

Closes #8

Author: greenkeeper[bot]

package.json

@@ -49,7 +49,7 @@
     "codecov": "^3.0.0",
     "esdoc": "^1.0.4",
     "esdoc-standard-plugin": "^1.0.0",
-    "eslint": "^4.14.0",
+    "eslint": "^6.4.0",
     "eslint-config-airbnb-base": "^12.1.0",
     "eslint-plugin-import": "^2.8.0",
     "mocha": "^5.0.1",