Run sitemap refresh task on restart.

As part of [adding sitemap support to Supermarket](https://github.com/opscode/supermarket/pull/394), I thought it would make sense to run the sitemap refresh command after restart.

I am not 100% sure if this is the correct way to handle this, so I am just throwing this out there. It is worth noting that the Supermarket PR is using [the sitemap_generator gem](https://github.com/kjvarga/sitemap_generator) to accomplish this.