fix container names in default config

sloth/conf/default_config.py

@@ -21,8 +21,8 @@ INSERTERS = {
 }
 
 CONTAINERS = (
-    ('*.json',   'sloth.annotations.container.JSONContainer'),
-    ('*.yaml',   'sloth.annotations.container.YAMLContainer'),
+    ('*.json',   'sloth.annotations.container.JsonContainer'),
+    ('*.yaml',   'sloth.annotations.container.YamlContainer'),
     ('*.pickle', 'sloth.annotations.container.PickleContainer'),
 )