ruby.rubygems: add meta.mainProgram

pkgs/development/interpreters/ruby/rubygems/default.nix

···
     changelog = "https://github.com/rubygems/rubygems/blob/v${version}/CHANGELOG.md";
     homepage = "https://rubygems.org/";
     license = with licenses; [ mit /* or */ ruby ];
+    mainProgram = "gem";
     maintainers = with maintainers; [ zimbatm ];
   };
 }