chore(package): update xo to version 0.20.0

Author: greenkeeper[bot]

package.json

@@ -46,6 +46,6 @@
     "eslint-config-xo-lukechilds": "^1.0.0",
     "nyc": "^11.0.3",
     "this": "^1.0.2",
-    "xo": "^0.19.0"
+    "xo": "^0.20.0"
   }
 }