Defining a specific YAML loader (#155)

As per using one of the "'sugar' methods" mentioned in https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation#how-to-disable-the-warning

disco/util/serializer.py

@@ -21,8 +21,8 @@ class Serializer(object):
 
     @staticmethod
     def yaml():
-        from yaml import load, dump
-        return (load, dump)
+        from yaml import full_load, dump
+        return (full_load, dump)
 
     @staticmethod
     def pickle():