fix pyyaml version since newer version would cause error (#922)

Summary:
I found that newer version of pyyaml cause error while parsing yaml files. Fix the pyyaml version can solve this problem.
Reference issue: #840
Pull Request resolved: #922

Reviewed By: rbgirshick

Differential Revision: D16483989

Pulled By: ir413

fbshipit-source-id: 427013e8fd045184401b5669af18314d5e8ecba1

requirements.txt

@@ -1,5 +1,5 @@
 numpy>=1.13
-pyyaml>=3.12
+pyyaml==3.12
 matplotlib
 opencv-python>=3.2
 setuptools