Add ruby-2.0.0-p648 to the list

[filbranden]

It was added to toolcache in commit ruby/ruby-builder@9fa2773.

Tested that it works, using both ubuntu-latest and macos-latest:
https://github.com/filbranden/vimgolf/actions/runs/887034477

Fixes #181.

[eregon]

Thanks for the PR, could you also add it in the README (just as 2.0.0), and add 2.0.0 in the CI workflow?

[filbranden]

@eregon I updated the PR, pushed a follow up commit updating the README and adding the version to the CI.

My own CI run failed to resolve macos-11.0, not sure why that happened, since the weekly run here worked just fine...
https://github.com/filbranden/setup-ruby/actions/runs/893761034

In any case, doesn't look related to these changes, and the runs on 2.0 all succeeded (with the exception of the macos-11.0 one, so not completely sure whether that one is fine or not.)

Let me know if you need anything else from me on this PR, happy to follow up!

Cheers,
Filipe

[filbranden]

Ah! "The macOS 11 virtual environment is currently provided as a private preview only." so that explains it... Anyways, please run it on this PR to confirm it works as expected. Cheers!