Lock dependencies for Ruby 2.0 compatibility.

gemfiles/Gemfile.rails3.2

@@ -28,7 +28,9 @@ group :test do
     gem 'sqlite_ext', '~> 1.5.0'
   end
 
-  gem 'webmock'
+  gem 'public_suffix', '2.0.5' # webmock dependency
+  gem 'addressable', '2.5.2' # webmock dependency
+  gem 'webmock', '3.3.0'
 
   platforms :ruby do
     gem 'pg', '~> 0.11'