Pin ffi to < 0.13 for windows

ffi 0.13 crashes Ruby on windows - see ffi/ffi#784

Signed-off-by: James Stocks <jstocks@chef.io>

omnibus.gemspec

@@ -24,6 +24,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency "aws-sdk-s3",       "~> 1"
   gem.add_dependency "chef-sugar",       ">= 3.3"
   gem.add_dependency "chef-cleanroom",   "~> 1.0"
+  gem.add_dependency "ffi",              "< 1.13"           # 1.13 does not work on Windows: https://github.com/ffi/ffi/issues/784 
   gem.add_dependency "ffi-yajl",         "~> 2.2"
   gem.add_dependency "mixlib-shellout",  ">= 2.0", "< 4.0"
   gem.add_dependency "ohai",             ">= 13", "< 17"