chore(package): update xo to version 0.21.1

Closes #31

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "remark-usage": "^6.0.0",
     "remark-validate-links": "^7.0.0",
     "tape": "^4.6.0",
-    "xo": "^0.18.0"
+    "xo": "^0.21.1"
   },
   "xo": {
     "space": 4,