chore(package): update babel-eslint to version 9.0.0

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
   },
   "homepage": "https://github.com/FullHuman/purgecss-webpack-plugin#readme",
   "devDependencies": {
-    "babel-eslint": "^8.2.3",
+    "babel-eslint": "^9.0.0",
     "babel-plugin-external-helpers": "^6.22.0",
     "babel-plugin-syntax-object-rest-spread": "^6.13.0",
     "babel-plugin-transform-object-rest-spread": "^6.26.0",