(PUP-2777) Fix Gemfile test for x64 platform

 - Previously it was assumed that the :x64_mingw platform was only
   defined in the Bundler DSL as a valid platform on a x64 Ruby.
   However, this is not actually the case.  Due to a coincidence, during
   testing, Bundler 1.3.5 was used on Ruby 1.9.3 x86 and that old
   Bundler version didn't have a :x64_mingw platform.  Upon upgrading
   to Bundler 1.6.3, :x64_mingw became defined on Ruby 1.9.3 x86.
   So the check was adjusted to what Bundler uses internally to
   determine if the current platform is running on x64.