Bugfix: Fixing python-etcd version, as behaviour has changed in newer version.

Our current master branch doesn't pass the code coverage test, due to behaviour changes
in upstream python-etcd.
As a bandaid, fix the version for now.

Reference build fail:

https://travis-ci.org/zalando/patroni/jobs/98470121

requirements-py2.txt

@@ -6,6 +6,6 @@ PyYAML
 requests
 six >= 1.7
 kazoo>=2.2.1
-python-etcd>=0.4.1
+python-etcd==0.4.1
 click>=4.1
 prettytable>=0.7

requirements-py3.txt

@@ -6,6 +6,6 @@ PyYAML
 requests
 six
 kazoo>=2.2.1
-python-etcd>=0.4.1
+python-etcd==0.4.1
 click>=4.1
 prettytable>=0.7