use separate gemspec for mingw

- allows us to explicitly version pin in the gemspec on gem installs
  rather than getting different behavior from 'gem install' and
  'bundle install' (gemspec vs. Gemfile).
- the mingw deps had been in the Gemfile because it needed to be a
  group since gemspecs do not have groups.  this builds two different
  gems for publication to rubygems instead.
- see mixlib-shellout for the prototype of where this has been done
  (successfully).