Return to readline version 6.x in Homebrew to fix Postgresql?

Just reinstall postgresql94 package, the PostgreSQL 9.4 will use the new readline package:

brew reinstall postgresql94

To answer the specific question.

You can switch between version of installed brew packages which are still around locally as explained in this question: Homebrew install specific version of formula?

To downgrade to readline 6.x the following command worked for me:

brew switch readline 6.3.8