Update TruffleRuby dependencies
Update TruffleRuby dependencies

* TruffleRuby 19.3+ uses an internal LLVM toolchain and no longer depends on LLVM.
* The full dependencies are listed at
  https://github.com/oracle/truffleruby/blob/master/README.md#dependencies
* Previous versions of TruffleRuby detect if LLVM is missing and print
  a nice error message, so it is not an issue to remove the dependency
  for older versions.

postmodern/ruby-install,postmodern/ruby-install