chore(package): update fs-extra to version 8.1.0

Closes #18

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
   },
   "devDependencies": {
     "express": "~4.16.3",
-    "fs-extra": "~6.0.1",
+    "fs-extra": "~8.1.0",
     "mocha": "~5.2.0",
     "sinon": "~6.0.0"
   }