Add license information to gemspec.

Author: frodsan

actionpack-action_caching.gemspec

@@ -13,6 +13,7 @@ Gem::Specification.new do |gem|
   gem.executables   = gem.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
   gem.require_paths = ['lib']
+  gem.license       = 'MIT'
 
   gem.add_dependency 'actionpack', '>= 4.0.0', '< 5.0'