We should generate the protobuf code with an old compiler

[drolando]

When importing py_zipkin on a system with an older protobuf library version (doesn't need to be a different major version, minor versions are enough) we might get errors due to the code using newer features.

Installing py-zipkin 0.18.2 in a system with protobuf==3.5.2 causes TypeError: __init__() got an unexpected keyword argument 'serialized_options'.
From protocolbuffers/protobuf#4716:

This error happens when new generated code is being used with an old runtime, That's a configuration we don't support. Upgrade protobuf runtime to newer version later than your protoc version can solve the error.
For compatibility, we support older generated code with newer protobuf runtime.

We should probably regenerate the protobuf code with an older compiler. @adriancole what do you do in java-land? I'd guess you'll have the same issue.

[codefromthecrypt]

the only problem we had was caused by depending on a google type "empty" but that was reverted in api 2.1 which should be released soon. Can you try with the file from here? https://dist.apache.org/repos/dist/dev/incubator/zipkin/zipkin-api/0.2.1/