chore(package): update xo to version 0.31.0

Closes #10

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
   },
   "devDependencies": {
     "eslint-config-prettier": "^2.9.0",
-    "xo": "^0.20.2"
+    "xo": "^0.31.0"
   },
   "xo": {
     "spaces": 2,