Pinning networkx to 2.0
Pinning networkx to 2.0

Summary:
Cause 2.1 moved bellman_ford, and scikit-image will install the most recent networkx by default
Closes https://github.com/caffe2/caffe2/pull/1944

Reviewed By: pietern

Differential Revision: D6966299

Pulled By: pjh5

fbshipit-source-id: 71ad387cb4a2b22cde3b87e6665977da6b4c428e

caffe2/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2