update rack dependency to fix vulnerability

https://groups.google.com/forum/#!topic/ruby-security-ann/T4ZIsfRf2eA

Author: kshsieh

prerender_rails.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
-  spec.add_dependency 'rack', '~> 2.0.6'
+  spec.add_dependency 'rack', '~> 2.2.2'
   spec.add_dependency 'activesupport', '>= 0'
 
   spec.add_development_dependency "bundler", "~> 1.3"