chore(package): update eslint to version 6.3.0

Closes #38

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
     "@babel/preset-env": "^7.2.3",
     "@google-cloud/functions-emulator": "^1.0.0-beta.5",
     "babel-eslint": "^10.0.1",
-    "eslint": "^5.12.1",
+    "eslint": "^6.3.0",
     "eslint-plugin-promise": "^4.0.1",
     "firebase-bolt": "^0.8.4",
     "firebase-tools": "^6.3.1",