Freshen instead of upgrade on RPM installs

Addresses issue #22. 

If the installed version of chef-client matches the omnibus_updater download,
upgrade (-U) will exit with an error on the first-run.

This changes `rpm --upgrade` to `rpm --freshen`, which doesn't exit with a non-zero status code if the same package is already installed, and fixes this problem.

I'm unsure if people ever use omnibus_updater to do an initial chef-client install -- this will break that use-case. README isn't clear if that's in scope for the cookbook.