(maint) unpin puppet_agent fixture

Author: h0tw1r3

.fixtures.yml

@@ -4,6 +4,4 @@ fixtures:
     "cron_core": "https://github.com/puppetlabs/puppetlabs-cron_core.git"
     "facts": "https://github.com/puppetlabs/puppetlabs-facts.git"
     "provision": "https://github.com/puppetlabs/provision.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'