chore(package): update xo to version 0.20.1 (#14)

Closes #13

Author: lukechilds

package.json

@@ -50,6 +50,6 @@
     "nyc": "^11.0.3",
     "requirable": "^1.0.4",
     "this": "^1.0.2",
-    "xo": "^0.19.0"
+    "xo": "^0.20.1"
   }
 }