Remove outdated pin

Author: LukasAud

.fixtures.yml

@@ -5,6 +5,4 @@ fixtures:
   repositories:
     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'