Bump xo from 0.18.2 to 0.21.1

Bumps [xo](https://github.com/xojs/xo) from 0.18.2 to 0.21.1.
- [Release notes](https://github.com/xojs/xo/releases)
- [Commits](xojs/xo@v0.18.2...v0.21.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

package.json

@@ -58,7 +58,7 @@
     "jest-cli": "^23.0.0",
     "pre-commit": "^1.2.2",
     "prettier": "^1.9.2",
-    "xo": "^0.18.0"
+    "xo": "^0.21.1"
   },
   "pre-commit": [
     "prettier"