add yum module downgrade support

Allows the yum module to downgrade an RPM if it's older than the currently installed version.

From the ref thread below, we discuses adding an optional version= param and mentioning that you can specify the version in the name. I didn't end up doing it that way because version is a bit ambiguous due to RPM's complex notion of version (i.e. epoch, rel, ver). Using the regular spec notation (it's an expression actually) would be more intuitive I believe, and there's examples in the module already. But am happy to adjust as needed. This is prob more of a bug fix than a feature add.

Ref thread:
https://groups.google.com/forum/#!searchin/ansible-project/yum$20downgrade/ansible-project/P_Rp4fVJYHk/ZettkjxKBlcJ