switches from our private `windows_os?` method to the one in RSpec::Support
JRuby 2.0.0 currently produces no warning, but this mode is still experimental, so skip instead.
Merge pull request #1715 from rspec/update_warnings

JRuby 2.0.0 currently produces no warning
Merge pull request #1714 from rspec/switch_core_to_use_os_support

Tidy up windows support.
returning command failure message only when in verbose mode