Lock to Ruby 1.9.3-compatible gem version.

gemfiles/Gemfile.ruby1.9.3

@@ -11,6 +11,7 @@ group :development, :test do
   gem 'sqlite_ext', '~> 1.5.0'
   gem 'pg'
   gem 'mysql2', '~> 0.3.11'
+  gem 'public_suffix', '1.4.6'
 
   # i18n gem >=0.7.0 does not work with Ruby 1.9.2
   gem 'i18n', '0.6.1'