Support for short version syntax

[solnic]

It would be very helpful if this action supported short version syntax too, examples:

• ruby-2.6
• jruby-9.2.9
• truffleruby-19.3

[eregon]

Indeed, that should be easy because there is a list of versions at https://github.com/eregon/ruby-install-builder/blob/metadata/versions.json
I'll try to implement it today.

[eregon]

I implemented it in #2 and updated the README.