chore(package): update xo to version 0.28.0

Closes #30

Author: greenkeeper[bot]

package.json

@@ -42,7 +42,7 @@
     "electron-webpack": "1.9.0",
     "webpack": "^3.6.0",
     "webpack-dev-server": "^2.9.2",
-    "xo": "^0.18.2"
+    "xo": "^0.28.0"
   },
   "xo": {
     "esnext": true,