Update xo requirement from ^0.17.1 to ^0.24.0

Updates the requirements on [xo](https://github.com/xojs/xo) to permit the latest version.
- [Release notes](https://github.com/xojs/xo/releases)
- [Commits](xojs/xo@v0.17.1...v0.24.0)

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

package.json

@@ -46,6 +46,6 @@
     "istanbul": "^0.4.5",
     "mocha": "^3.2.0",
     "should": "^11.1.2",
-    "xo": "^0.17.1"
+    "xo": "^0.24.0"
   }
 }