fixtures: pull puppetlabs-puppet_agent from the main branch

It was originally like this in
43a4d26 (#864), but changed to a
version pin in 89721b1 (#1095). I
think the reasoning in #1095 is not valid anymore, so we should set
this back to use main.

Author: kenyon

.fixtures.yml

@@ -3,9 +3,7 @@ fixtures:
     "stdlib":
       "repo": "https://github.com/puppetlabs/puppetlabs-stdlib.git"
     facts: 'https://github.com/puppetlabs/puppetlabs-facts.git'
-    puppet_agent:
-      repo: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
-      ref: v4.13.0
+    puppet_agent: 'https://github.com/puppetlabs/puppetlabs-puppet_agent.git'
     provision: 'https://github.com/puppetlabs/provision.git'
   symlinks:
     "apt": "#{source_dir}"