Add libvips dependency to gemspec which is respected by RubyInstaller on Windows
Add libvips dependency to gemspec which is respected by RubyInstaller on Windows

This automatically installs the mingw package "libvips" at gem install,
so that ruby-vips can be used very easy.

The mechanism is described in:
  https://github.com/oneclick/rubyinstaller2/wiki/For-gem-developers#user-content-msys2-library-dependency

jcupitt/ruby-vips,jcupitt/ruby-vips