fix pyyaml version since newer version would cause error (#922)
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: https://github.com/facebookresearch/Detectron/issues/840
Pull Request resolved: https://github.com/facebookresearch/Detectron/pull/922

Reviewed By: rbgirshick

Differential Revision: D16483989

Pulled By: ir413

fbshipit-source-id: 427013e8fd045184401b5669af18314d5e8ecba1

facebookresearch/Detectron,facebookresearch/Detectron,facebookresearch/Detectron