Have `rbenv install --list` output match `ruby-build --definitions`
Have `rbenv install --list` output match `ruby-build --definitions`

This is for simplicity and usability in scripts.

Fixes #1170

makandra/ruby-build,nodenv/node-build,sstephenson/ruby-build,rbenv/ruby-build,ryanfb/ruby-build,OiNutter/node-build,dark-panda/ruby-build,rbenv/ruby-build,dark-panda/ruby-build