Clean up `jenkins_run_tests.bat`

This change ensures the bat file is in sync with what we have in
opscode/chef.
Silence `:rubygems` Bundler deprecation notice.

Message from Bundler before this change:

  The source :rubygems is deprecated because HTTP requests are insecure.

  Please change your source to 'https://rubygems.org'; if possible, or
  'http://rubygems.org'; if not.
Merge pull request #32 from opscode/fix-windows-tests

Fix windows tests