[celery] fix imports

Author: Kyle-Verhoog

ddtrace/contrib/celery/signals.py

@@ -5,7 +5,7 @@
 from celery import registry
 
 from . import constants as c
-from .util import (
+from .utils import (
     tags_from_context,
     retrieve_task_id,
     attach_span,

tests/contrib/celery/test_utils.py

@@ -2,7 +2,7 @@
 
 from nose.tools import eq_, ok_
 
-from ddtrace.contrib.celery.util import (
+from ddtrace.contrib.celery.utils import (
     tags_from_context,
     retrieve_task_id,
     attach_span,