Modify setup.py version syntax to address #5464

Some users report that even with version <4.0 specified in setup.py,
the 4.0-rc builds of kombu and celery were still being downloaded.
This patch uses a different version syntax as suggested in a Stack
Overflow thread.

https://stackoverflow.com/a/14405269

setup.py

@@ -56,8 +56,8 @@ install_requires = [
     'pytest>=2.3.1',
     'pytest-xdist',
     'werkzeug>=0.7',
-    'celery>=3.0,<4.0',
-    'kombu<4.0',
+    'celery>=3.0,<4.0a0',
+    'kombu<4.0a0',
     'jinja2',
     'Babel>=1.3',
     'WebTest>=2.0.18',