Enable drop / reload views on rake db:migrate

If you `include Etc::Migration` in any migration, the following will be
executed inside a transaction:

1. rake db:etc:drop
2. migration
3. rake db:etc:load

@bigeasy please let me know if you think it would be better to do this for all migrations (monkeypatching `ActiveRecord::Migration` instead of including a module in a specific migration).