chore(package): update xo to version 0.31.0

Closes #84

Author: greenkeeper[bot]

package.json

@@ -41,7 +41,7 @@
     "husky": "^1.2.0",
     "jest": "^23.6.0",
     "lint-staged": "^8.1.0",
-    "xo": "^0.23.0"
+    "xo": "^0.31.0"
   },
   "jest": {
     "testEnvironment": "node",