-preview2 of the Ruby 2.4  was released by upstream and here is the
test build for you (ARM is not finished yet, but I hope it will pass):
You can find the necessary changes in private-ruby-2.4 branch of dist-git.
When I did the last snapshot, I mentioned, that Tcl/Tk was moved into
gem. But upstream took step further and dropped the Tcl/Tk support from
the Ruby stdlib entirely. The tk gem is the only option now. Hence I
dropped the subpackage as well. The only caveat is that there is not
good place to obsolete this package. I figured out that I place the
obsoletes into ruby-libs to remove the ruby-tcltk package from the
system, but if you have any better idea, please let me know.