chore(package): update eslint to version 6.5.0

Closes #28

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "@babel/polyfill": "^7.0.0",
     "babel-eslint": "^10.0.1",
     "coveralls": "^3.0.2",
-    "eslint": "^5.7.0",
+    "eslint": "^6.5.0",
     "eslint-plugin-prefer-let": "^1.0.1",
     "expect": "^24.1.0",
     "mocha": "^6.0.0",