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

Closes #20

Author: greenkeeper[bot]

package.json

@@ -21,7 +21,7 @@
   "devDependencies": {
     "coveralls": "^3.0.1",
     "documentation": "^8.0.0",
-    "fs-extra": "^6.0.0",
+    "fs-extra": "^8.1.0",
     "level-browserify": "^1.1.2",
     "nyc": "^12.0.1",
     "standard": "^11.0.1",