shorter git command chains

Add truffleruby-dev to install the latest TruffleRuby nightly build

* Otherwise, existing files in the prefix could easily conflict and
  potentially cause very hard to debug errors.
* This is particularly important for *-dev builds which might be
  repeatedly installed to the same directory.

* Uses the latest nightly, which is automatically built.

ruby 2.3 EOL

Update so Ruby 1.9.3 thru 2.3, and rbx can be built with OpenSSL 1.0.2u

Create definition file truffleruby-19.3.0.2

Add definition for Ruby 2.7.0

Ruby 2.7.0 has been released.
https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/

add ruby 2.7.0-rc2

Add definition for Ruby 2.7.0-rc1

Ruby 2.7.0-rc1 has been released.
https://www.ruby-lang.org/en/news/2019/12/17/ruby-2-7-0-rc1-released/