Add Benchmark to activemerchant test suite

Author: hannahramadan

test/multiverse/suites/activemerchant/Envfile

@@ -16,6 +16,7 @@ def gem_list(activemerchant_version = nil)
     gem 'activesupport'
     gem 'nokogiri'
     gem 'minitest', '~> 5.1.0'
+    gem 'benchmark'
 
     # Need to load newrelic_rpm after ActiveMerchant Gateways are required
     gem 'newrelic_rpm', :require => false, :path => File.expand_path('../../../../')