Fix pg gem installation on rhel family
Fix pg gem installation on rhel family

* See https://github.com/bigbinary/boxy/issues/92.

* Bundler cannot find `pg_config` on path
and `bundle install pg` fails.

* Puts `pg_config` on path so that bundler
can see it.

bigbinary/boxy-rails,bigbinary/boxy-rails,bigbinary/boxy-rails