chore(package): update eslint to version 6.4.0

Closes #16

Author: greenkeeper[bot]

package.json

@@ -24,7 +24,7 @@
     "babel-preset-es2015": "^6.24.1",
     "chai": "^4.1.2",
     "codecov": "^3.0.4",
-    "eslint": "^5.2.0",
+    "eslint": "^6.4.0",
     "eslint-config-airbnb-base": "^13.0.0",
     "eslint-config-prettier": "^3.0.0",
     "eslint-plugin-import": "^2.13.0",