remove outdated pin

Author: LukasAud

.fixtures.yml

@@ -1,9 +1,7 @@
 fixtures:
   repositories:
     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'
     stdlib:  https://github.com/puppetlabs/puppetlabs-stdlib.git
     registry: https://github.com/puppetlabs/puppetlabs-registry.git
     provision: 'https://github.com/puppetlabs/provision.git'