Update the ZeroRPC middleware for zerorpc-python >= 0.4.0

Hello,

We are releasing a new version of [zerorpc-python](https://github.com/dotcloud/zerorpc-python/tree/v0.4.0) with a different middleware API.

This pull requests update the Raven's zerorpc middleware to the new API and fixes a bug with how traceback are handled in PUB/SUB or PUSH/PULL zerorpc servers.

Thanks