Add definition for Ruby 2.7.0-preview3
Add definition for Ruby 2.7.0-preview3

Ruby 2.7.0-preview3 has been released.
https://www.ruby-lang.org/en/news/2019/11/23/ruby-2-7-0-preview3-released/

And this PR has been verified using a modified checksum.
https://github.com/ruby/www.ruby-lang.org/pull/2284

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