chore(package): update xo to version 0.28.2

Closes #20

Author: greenkeeper[bot]

package.json

@@ -54,6 +54,6 @@
     "eslint-plugin-flowtype": "2.46.3",
     "eslint-plugin-react": "7.8.0",
     "jest": "22.4.3",
-    "xo": "0.21.0"
+    "xo": "0.28.2"
   }
 }