Now depend on Kombu 2.4.3

Kombu 2.4.2 doesn't work with Celery 3.0.7.

requirements/default.txt

@@ -1,3 +1,3 @@
 billiard>=2.7.3.12
 python-dateutil>=1.5,<2.0
-kombu>=2.4.2,<3.0
+kombu>=2.4.3,<3.0

setup.cfg

@@ -18,5 +18,5 @@ requires = uuid
            importlib
            billiard>=2.7.3.12
            python-dateutil >= 1.5
-           kombu >= 2.4.2
+           kombu >= 2.4.3
            ordereddict