chore(package): update xo to version 0.20.2

Closes #9

Author: greenkeeper[bot]

package.json

@@ -34,7 +34,7 @@
   },
   "devDependencies": {
     "semantic-release": "^12.2.2",
-    "xo": "^0.18.2"
+    "xo": "^0.20.2"
   },
   "xo": {
     "space": 4,