rake db:migrate skips migrations with older version numbers than current

Not sure if this is considered a bug, but the behavior is surprising enough that I think it's worth changing. When using timestamped migration version numbers, running db:migrate doesn't run migrations with versions less than the highest previously applied migration. Here's an example using small integers rather than big timestampy numbers:

1. Start with migration versions in the db/migrate directory: 1, 2, 5, 7, 10, 12.
   
2. run rake db:migrate # => runs all migrations
   
3. Add migrations to directory with versions: 6, 8, 15, 16.
   
4. run rake db:migrate # => migrations 15 & 16 are run, but 6 & 8 are skipped.

I think it is less surprising to run all the migrations, not just the ones with the version number > 12.

I can do a patch for this if people think it's the right thing to do.