Fix for art migrate:rollback error, and a proposal

The second commit in this pull request fixes the error you get on a fresh install if you try to rollback the migrations.

The last commit is more of a proposal. To nest the post slug routes in /blog/. This will help if other parts of the site need to parse URI segments as arguments, since the first one could be matching a post slug or vice versa. I think it makes things clearer since this is a base starter site.

Hope it helps :)

ps: sorry for the commits not done on my Git username, hadn't setup my email yet on the virtual machine ^^