warn user that installation with --system flag will fail if she's not root #464

[Mizou04]

fix for issue #464

[postmodern]

It's generally a bad practice to put additional logic in option parsing. The option parser should only parse the options. Any validation logic should happen once all of the options have been parsed. This prevents issues where options can be specified in different orders.

I feel like testing if $install_dir is writable is a better solution, as that will also catch edge-cases where the user is trying to explicitly install ruby to somewhere else on the system: ruby-install --install-dir /opt/ ruby, etc This guard logic could be added to pre_install or directly to bin/ruby-install.