Fix order in which we call the aliased create_table

At least under rails 4, the guts of create_table (with force: true) will
call drop_table.  The order in which the actions happened meant that we
would create the global_uid tables and then happily drop them.

this really only happened with force: true, in schema.rb.
Merge pull request #20 from zendesk/ben/fix_migration_extension

Fix order in which we call the aliased create_table