Update PyYAML version restriction according to requirement in docker-compose (#523)

Author: yangaws

setup.py

@@ -34,7 +34,7 @@ def read(fname):
 
 # Declare minimal set for installation
 required_packages = ['boto3>=1.9.55', 'numpy>=1.9.0', 'protobuf>=3.1', 'scipy>=0.19.0',
-                     'urllib3>=1.21', 'PyYAML>=3.2', 'protobuf3-to-dict>=0.1.5',
+                     'urllib3>=1.21', 'PyYAML>=3.2, <4', 'protobuf3-to-dict>=0.1.5',
                      'docker-compose>=1.23.0']
 
 # enum is introduced in Python 3.4. Installing enum back port