add postgres_version to fix libpq5 dependency issues

According to the [Postgresql FAQ](http://wiki.postgresql.org/wiki/Apt/FAQ#I_want_to_try_the_beta_version_of_the_next_PostgreSQL_release), the libpq5 dependency is mapped to support for the "main archive" Postgresql versions. Therefore, I ran into issues when installing version `9.4`. By adding the `postgresql_version`, I'm able to move between Postgres versions since I'm overriding the `libpq5` dependency setup.

This seems to be a common issue referenced by other issues,
http://www.postgresql.org/message-id/20130808001257.GA15451@llserver.lakeliving.com and http://www.postgresql.org/message-id/521BE88E.50300@agliodbs.com.