Refactor the installation code

Move the code for installing the ruby gem (using the gem
provider) in its own class.
Add rpm install method for RedHat based systems
Add missing dependency to the rpm package resource
Remove purgedirs support

r10k never seemed to actually use the purgedirs option in
the configuration. So it was removed from the documentation some
time ago. See adrienthebo/r10k@ea4495aba839d0280bd0ce6c47b63650393fbc68.
Merge remote-tracking branch 'origin/remove-purgedirs'