Include zoneinfo only for freebsd, window

Fix following error when using MAC OSX:

`x64_mingw` is not a valid platform. The available options are: [:ruby,
:ruby_18, :ruby_19, :ruby_20, :mri, :mri_18, :mri_19, :mri_20, :rbx,
:jruby,
:mswin, :mingw, :mingw_18, :mingw_19, :mingw_20]