Tweak build command

Author: gaearon

package.json

@@ -72,7 +72,7 @@
     "url": "git+https://github.com/reactjs/reactjs.org.git"
   },
   "scripts": {
-    "build": "gatsby build",
+    "build": "gatsby clean && gatsby build",
     "check-all": "npm-run-all prettier generate-ids --parallel lint flow",
     "ci-check": "npm-run-all prettier:diff --parallel lint flow",
     "dev": "gatsby develop -H 0.0.0.0",