fix(package): update fs-extra to version 5.0.0

Closes #18

Author: greenkeeper[bot]

package.json

@@ -52,7 +52,7 @@
     "postcss-polymer-loader"
   ],
   "dependencies": {
-    "fs-extra": "4.0.2",
+    "fs-extra": "5.0.0",
     "loader-utils": "1.1.0",
     "parse5": "3.0.3",
     "postcss": "6.0.14",