Fix for migrate connection error

Fixes issue #741 
This issue is a real show-stopper in 1.7. It completely breaks migrations for us. We are currently having to run off this branch of the 1.7.1 tag in order to use Propel.

Credit for fix: https://github.com/jh-ism-online-webmaster/Propel/commit/846640a8d00a01c462c3dc78969afa2931e75b54