Warn about missing migrations

Every time users boot their apps they would see a warning like:

    [spree] Missing migrations. Run `bundle exec rake railties:install:migrations` to get them.

Would you guys think this is a valid / useful idea? Or would this just bring too much overhead and won't ever work properly anyway (aka bad stupid idea)? I think it's pretty common to forget about copying migrations after upgrading spree or some extension and stop a bit yesterday to think about what we could do to remember users while they're launching their apps.

Note: this code is probably is probably broken. I opened the PR more as a way to get feedback on the idea. Maybe we could move on from here to a better implementation, put it in another gem or just forget about it.

no :beers: this weekend