Bound kafka client version

Starting with 2.0.4, the errors module does not expose a tuple of
retriable errors: instead there is an attribute on each error type.

Making journalpump compatible with both the old and new version is
outside the scope of this change, so bound the kafka client version.

Author: rdunklau

setup.py

@@ -10,7 +10,7 @@
     packages=find_packages(exclude=["test", "systest"]),
     extras_require={},
     install_requires=[
-        "kafka-python",
+        "kafka-python<2.0.4",
         "requests",
         "websockets",
         "aiohttp-socks",