Fix loading error on Windows (#315)
Fix loading error on Windows (#315)

It causes the error if it requires RMagick on Windows with recent [RubyInstaller](https://rubyinstaller.org).

To solve this error, you need to copy the DLL accoding to article in https://medium.com/ruby-on-rails-web-application-development/install-rmagick-gem-on-windows-7-8-10-imagemagick-6-9-4-q16-hdri-5492c3fef202#f956

This Pull Request fix this error accoding to https://github.com/oneclick/rubyinstaller2/wiki/For-gem-developers

With this patch, RMagick works well on Windows.
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick