Now depends on billiard 2.7.3.19 (but always ignore 3.x)

requirements/default.txt

@@ -1,3 +1,3 @@
-billiard>=2.7.3.18
+billiard>=2.7.3.19,<3.0
 python-dateutil>=1.5,<2.0
 kombu>=2.5.3,<3.0

setup.cfg

@@ -14,6 +14,6 @@ all_files = 1
 upload-dir = docs/.build/html
 
 [bdist_rpm]
-requires = billiard >= 2.7.3.18
+requires = billiard >= 2.7.3.19
            python-dateutil >= 1.5
            kombu >= 2.5.3