Avoid running XO on older Node.js versions

Author: novemberborn

package.json

@@ -13,7 +13,7 @@
     "node": ">=0.10.0"
   },
   "scripts": {
-    "test": "xo && ava",
+    "test": "node xo.js && ava",
     "preversion": "WRITE_EXAMPLES=1 npm run test && git add example-output.md"
   },
   "files": [

xo.js

@@ -0,0 +1,10 @@
+#!/usr/bin/env node
+/* eslint-disable import/no-unassigned-import */
+'use strict';
+
+var major = Number(process.version.match(/^v(\d+)/)[1]);
+if (major >= 4) {
+	require('xo/cli');
+} else {
+	console.warn('Linting requires Node.js >=4');
+}