Manually install ffi on Windows to make tests pass

Fabrizio Monti · sds · commit ed9f43245bfa · 2019-06-28T21:17:31.000-07:00
diff --git a/Gemfile b/Gemfile
@@ -13,3 +13,5 @@ gem 'coveralls', '~> 0.8'
 
 # Pin RuboCop for Travis builds.
 gem 'rubocop', '0.54.0'
+
+gem 'ffi' if Gem.win_platform?
