Update to install_red_hat_linux_stable_deps function to resolve issue 198.

This pull addresses concerns I raised in [issue 198](https://github.com/saltstack/salt-bootstrap/issues/198) regarding how the bootstrap errors out without a useful message for redhat users. Due to the way that the rhn-channel command operates (requiring a user/password to join to a channel) I've simply added in an error explaining that users need to enable the optional repo. At some point it might be good to expand on this, but I don't see a good way to do it right now.