Redhat or RedHat

_system_name can be `RedHat` on later versions of RHEL
Merge pull request #2682 from kmcgrath/master

Redhat or RedHat
simplify redhat check, update #2682
scaffold for #2673
use more of __rvm_version_sort, fix #2676
reworked file_exists_at_url to detect double --insecure and prevent weired fails, fix #2640, closes #2661
replace process when rvm .. do .. is called via PATH, update #2683
eliminate duplication in jruby requiremnts for ubuntu