Constrain python-daemon < 2.1.0

The latest python-deamon has broken zuul. Constrain zuul below
the last release.

Change-Id: I0af0dfbf684de65ee140c69cf564b4f95e05e0ba
diff --git a/requirements.txt b/requirements.txt
index c682999..6318a59 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,7 +7,7 @@
 paramiko>=1.8.0
 GitPython>=0.3.3
 ordereddict
-python-daemon>=2.0.4
+python-daemon>=2.0.4,<2.1.0
 extras
 statsd>=1.0.0,<3.0
 voluptuous>=0.7