fix(package): update readdirp to version 3.1.1

Closes #127

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
     "mkdirp": "^0.5.1",
     "optimist": "^0.6.1",
     "prettier": "^1.17.0",
-    "readdirp": "^2.1.0",
+    "readdirp": "^3.1.1",
     "valid-url": "1.0.9",
     "winston": "^3.1.0"
   },