buildRubyGem: use a saner default version to fix #11805

Previously the gems defaulted to "ruby" as the name and
"${ruby-version}-${gem-name}-${gem-version}" as the version,
which was just insane.

https://github.com/NixOS/nixpkgs/issues/9771#issuecomment-141041414
Noone is reacting so it's high time to take at least some action.
/cc @cstrahan.

pkgs/development/interpreters/ruby/gem.nix

@@ -6,7 +6,7 @@ lib.makeOverridable (
 , ruby ? defs.ruby
 , rubygems ? (rubygemsFun ruby)
 , stdenv ? ruby.stdenv
-, namePrefix ? "${ruby.name}" + "-"
+, namePrefix ? "${lib.replaceStrings ["-"] ["_"] ruby.name}" + "-"
 , buildInputs ? []
 , doCheck ? false
 , dontBuild ? true