chore(package): update xo to version 0.20.1

Closes #34

Author: greenkeeper[bot]

package.json

@@ -67,6 +67,6 @@
     "standard-version": "4.3.0",
     "text-loader": "0.0.1",
     "webpack": "3.11.0",
-    "xo": "0.18.2"
+    "xo": "0.20.1"
   }
 }