Use bundler standalone for app veyor builds.

Our specs that check there are no load-time warnings are failing
due to a warning coming from bundler:

C:/Ruby193/lib/ruby/gems/1.9.1/gems/bundler-1.7.8/lib/bundler/source/rubygems.rb:191: warning: shadowing outer local variable - spec

By avoiding loading bundler we can avoid this warning.