Show HEROKU_POSTGRESQL_COLOR name when connecting via pg:psql

Show the name of the Heroku Postgres database you're connecting to:

```bash
$ heroku pg:psql HEROKU_POSTGRESQL_GRAY -a app
---> Connecting to HEROKU_POSTGRESQL_GRAY
psql (9.2.4, server 9.2.5)
SSL connection (cipher: DHE-RSA-AES256-SHA, bits: 256)
Type "help" for help.

ec2-54-***-***-5 => \q

$ heroku heroku pg:psql -a app
---> Connecting to HEROKU_POSTGRESQL_ROSE_URL (DATABASE_URL)
Expanded display is used automatically.
Timing is on.
psql (9.2.4)
SSL connection (cipher: DHE-RSA-AES256-SHA, bits: 256)
Type "help" for help.

ec2-54-***-***-169 => \q