Fix plugin agent pgbouncer

On Ubuntu 12.04 (and potentially others), trying to install `newrelic_plugin_agent[pgbouncer]` breaks b/c it cannot find `pg_config`.

Turns out either `libpq-dev` (for Debian/Ubuntu systems) or `postgresql-devel` (others) is needed for this to succeed.

See http://stackoverflow.com/a/12037133/133479 for details.

I've tested this using test-kitchen, tests well.