chore(package): update xo to version 0.31.0

Closes #33

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
   "devDependencies": {
     "ava": "^2.3.0",
     "nyc": "^15.0.0",
-    "xo": "^0.25.3"
+    "xo": "^0.31.0"
   },
   "nyc": {
     "reporter": [