ruby-build 20200214
* Fix TruffleRuby 19.3.0.2 definition
* ruby 2.3 EOL
* use correct key named warn_eol
* remove unavailable jruby versions
* add back withdifferent download source
* add warn_eol
* ruby 2.4 security maintenance
* Add truffleruby-dev
* Uses the latest nightly, which is automatically built.
* Make sure to install to a clean prefix
* 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.