chore(package): update xo to version 0.20.0

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "rimraf": "^2.6.1",
     "semantic-release": "^12.2.2",
     "sinon": "^4.0.2",
-    "xo": "^0.18.2"
+    "xo": "^0.20.0"
   },
   "engines": {
     "node": ">=8.3"