Switch the Windows standalone build to Ruby 1.9.3.

Fixes the `illegal switch in RUBYOPT: -"` error when launching
`run.bat`. Bundler 1.3 is not compatible with Ruby 1.8 on Windows.

Bundler 1.3 quotes Windows include paths, which makes RUBYOPT look
like: `"-I/full/path/..."`. This works on Ruby 1.9, but Ruby 1.8
doesn't understand the quoted option and sees it as `ruby -"...`.
The fix is to upgrade Ruby or downgrade Bundler.

Fixes #1630, references #1559.