Use puppet yaml helper to workaround psych >4 breaking changes regarding positional arguments

Author: Gildas Fargeas

lib/puppet/provider/docker_compose/ruby.rb

@@ -28,7 +28,7 @@ def exists?
 
     # get merged config using docker-compose config
     args = [compose_files, '-p', name, 'config'].insert(3, resource[:options]).compact
-    compose_output = YAML.safe_load(execute([command(:dockercompose)] + args, combine: false), [Symbol])
+    compose_output = Puppet::Util::Yaml.safe_load(execute([command(:dockercompose)] + args, combine: false), [Symbol])
 
     containers = docker([
                           'ps',