Restore compatibility with protobuf2
Restore compatibility with protobuf2

Summary:
Addresses an issue with https://github.com/caffe2/caffe2/commit/417f74509e265d89511c740bcb0ff03d78d97fd0.
```
>               operators.append(proto.op.pop())
E               AttributeError: 'RepeatedCompositeFieldContainer' object has no attribute 'pop'
```
/cc jhcross
Closes https://github.com/caffe2/caffe2/pull/658

Reviewed By: dzhulgakov

Differential Revision: D5130382

Pulled By: salexspb

fbshipit-source-id: 34e0c39aad5f339c1aaa1506af3e7495193565f4

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