Add postgres-contrib dependency so that hstore tests can run

Without the `postgres-contrib` package, Postgres complains when Rails tries to create the test databases (since the hstore extension isn't available):

    vagrant@rails-dev-box:~/rails/activerecord$ rake postgresql:drop_databases
    vagrant@rails-dev-box:~/rails/activerecord$ rake postgresql:build_databases
    ERROR:  could not open extension control file "/usr/share/postgresql/9.1/extension/hstore.control": No such file or directory
    ERROR:  could not open extension control file "/usr/share/postgresql/9.1/extension/hstore.control": No such file or directory

I'm pretty sure this means the hstore tests are skipped*. This patch adds a Puppet dependency so that these tests can run.

*But I could totally be mistaken.