Release AR connections in test helper

Executing ```zeus rake test:prepare``` as the first command after ```zeus start``` fails complaining that the database is being accessed by other users.

However, after executing ```zeus test my_spec```, ```zeus rake test:prepare``` goes through.

I believe this is because the AR connection is associated with the test helper is not released.

This patch immediately releases the AR connection allowing ```zeus rake test:prepare``` to complete successfully.