Reorder #down in migration template so that it runs

Removing the columns first means the index will probably disappear so we can't then remove it, so we remove the index first.  Also use up and down, not self.up and self.down.
Ignore bundler artefacts
Merge pull request #4 from unboxed/make_migration_template_rollbackable

Reorder #down in migration template so that it runs