(MODULES-2452) Update Beaker Version

Update the Beaker version to use "2.20" which has an important fix for PE
installations.

Author: ferventcoder

Gemfile

@@ -38,7 +38,7 @@ end
 
 group :system_tests do
   gem 'beaker-rspec', *location_for(ENV['BEAKER_RSPEC_VERSION'] || '~> 5.1')
-  gem 'beaker', *location_for(ENV['BEAKER_VERSION'] || '~> 2.18')
+  gem 'beaker', *location_for(ENV['BEAKER_VERSION'] || '~> 2.20')
   gem 'beaker-puppet_install_helper',  :require => false
 end