(maint) Update 6.x branch name

We changed the branch name for the puppet6 stream from master to 6.x, so
update the acceptance tests to get the latest version from the correct
URL.

acceptance/tests/test_upgrade_puppet5_to_puppet6.rb

@@ -6,7 +6,7 @@
 test_name 'puppet_agent class: Upgrade agents from puppet5 to puppet6' do
   require_master_collection 'puppet6-nightly'
   exclude_pe_upgrade_platforms
-  latest_version = `curl http://builds.delivery.puppetlabs.net/passing-agent-SHAs/puppet-agent-master-version`
+  latest_version = `curl http://builds.delivery.puppetlabs.net/passing-agent-SHAs/puppet-agent-6.x-version`
 
   puppet_testing_environment = new_puppet_testing_environment