Add unique DB index to posts.guid.

Should have been there from the start, what the heck was I thinking? If you're hosting your own #pants instance and this migration is giving you trouble, you'll need to check your `posts` table for duplicate GUIDs and delete as many rows as necessary (there should only ever be one row per GUID.)

You can identify duplicate posts in your database through the following dumb statement in `rails console`:

    Post.group(:guid).pluck('guid, count(guid)').select { |i| i.last > 1 }