(FACT-567) support bundler x64

This commit makes it possible to do `bundle install` on both ruby 1.9 x86 and ruby 2.0 x64 on Windows, and have the correct set of gem dependencies installed.

Facter currently has a dependency on `windows-pr`, which depends on `win32-api`. Due to https://github.com/bundler/bundler/issues/3066, bundler will not install the `win32-api-1.5.1-universal` gem, and instead try to install `win32-api-1.4.8`, which requires the Ruby DevKit to be installed.

A future commit will remove the dependency on `windows-pr`, see https://jira.puppetlabs.com/browse/FACT-570
That's an interesting turn of events...back to the gemfile.