Update beaker pinning as release contains all our patches

Gemfile

@@ -21,7 +21,7 @@ group :development, :unit_tests do
 end
 
 group :system_tests do
-  gem 'beaker',        :require => false, :git => 'https://github.com/mcanevet/beaker', :branch => 'openstack'
+  gem 'beaker',        :require => false
   gem 'beaker-rspec',  :require => false
   gem 'serverspec',    :require => false
 end