Lock rack-cache to version 1.7.1 with Ruby 1.9.3.

gemfiles/Gemfile.ruby1.9.3

@@ -6,6 +6,7 @@ group :development, :test do
   gem 'bson_ext', :platforms => :ruby
   gem 'geoip'
   gem 'rubyzip'
+  gem 'rack-cache', '1.7.1'
   gem 'rails'
   gem 'sqlite3'
   gem 'sqlite_ext', '~> 1.5.0'