Set up Caffe2 versioning number
Set up Caffe2 versioning number

Summary:
Setting up a caffe2 versioning number per popular request.

The plan is to periodically update the version, with the current plan being
every other week. As a result I am setting the initial number to minor version
5 (since this is the 11th week of the year).

Reviewed By: salexspb

Differential Revision: D4725945

fbshipit-source-id: 9ff4c7e4a6341e22a5f1d4e25740705988cae84b

pietern/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,bwasti/caffe2,pietern/caffe2,xzturn/caffe2,caffe2/caffe2,bwasti/caffe2,bwasti/caffe2,sf-wind/caffe2,bwasti/caffe2,xzturn/caffe2,sf-wind/caffe2,davinwang/caffe2,davinwang/caffe2,pietern/caffe2,xzturn/caffe2,sf-wind/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,bwasti/caffe2,davinwang/caffe2,Yangqing/caffe2,pietern/caffe2