Do not use PyYAML 5.1 version

Error in 5.1:
YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.

Change-Id: I7d9bbcc33c37bb45d6126e5d96e59169623d6355
diff --git a/requirements.txt b/requirements.txt
index 2c964b2..6863949 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,5 @@
 paramiko>=1.16.0,<2.0.1
-PyYAML>=3.1.0
+PyYAML>=3.1.0,!=5.1
 six>=1.9.0
 cookiecutter
 jinja2==2.8.1