ruby.sh: cleanup gems on portable Ruby installation.

This avoids `brew style` and friends from getting upset when they
attempt to use the native gem extensions from a previous portable Ruby
or a system Ruby.

ABI? What's that?

Library/Homebrew/utils/ruby.sh

@@ -47,6 +47,7 @@ setup-ruby-path() {
         then
           odie "Failed to install vendor Ruby."
         fi
+        rm -rf "$vendor_dir/bundle/ruby"
         HOMEBREW_RUBY_PATH="$vendor_ruby_path"
       fi
     fi