Pin rexml at 3.1.9 for vimgolf gem testing
Pin rexml at 3.1.9 for vimgolf gem testing

The newer 3.1.9.1 fails on Ruby 2.0, so let's pin the version of this Gem in
order to still be able to test on Ruby 2.0.

igrigorik/vimgolf,igrigorik/vimgolf,igrigorik/vimgolf,igrigorik/vimgolf