Improve platform portability; other minor changes.
* Add --is-ruby-versions command-line option for testing whether a shimmable command (such as `ruby` or `bundle`) is in fact a ruby-versions shim.  ruby-versions returns a 0 exit code, whereas real commands will return a non-0 exit code.  (You will want to redirect STDERR to /dev/null.)
* Improve platform portability by not relying on arbitrary shebang line in ruby-versions.  Instead, during installation invoke via any locally available `ruby`, and change shebang line of script being installed to a real `ruby`.
* Minor clean-up of installation logic.
Merge branch 'master' of https://github.com/jmehnle/ruby-versions

Conflicts:
	bin/ruby-versions