document canonical name and move vars down to usage

Closes #20719.
Remove unused guard in canonical_name

Closes #20718.
use more env helpers
chruby 0.3.6

Closes #20729.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
ruby-install 0.2.0

Closes #20732.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>