Migrate down before each migration unit test

`migrate!` helper triggers a migration that is under the test, so we
need to revert it before each subsequent example. This means that we
need to place it in `before(:each, :migration)` hook.

We still want to migrate everything up *after* each *context*, so in
otherwords we need to place migrate up helper in `before(:context,
:migration)`.

spec/spec_helper.rb

@@ -132,7 +132,7 @@ RSpec.configure do |config|
     Sidekiq.redis(&:flushall)
   end
-  config.before(:context, :migration) do
+  config.before(:each, :migration) do
     schema_migrate_down!
   end