Update website on Travis when only new files are added (#8037)

This build didn't commit the new files because no existing files were changed:

https://travis-ci.org/facebook/react/jobs/169622664

This should work better.

.travis.yml

@@ -39,7 +39,7 @@ script:
         bundle exec rake release
         cd $GH_PAGES_DIR
         git status
-        if ! git diff-index --quiet HEAD --; then
+        if test -n "$(git status --porcelain)"; then
           git add -A .
           git commit -m "Rebuild website"
           git push origin gh-pages