Release the pin on protobuf

To resolve issue #339, dm-control [pins protobuf](https://github.com/google-deepmind/dm_control/blob/adbaa72a65ed63f00c3729ce0b5c2d83bec31c08/requirements.txt#L14) to a very old version. Could you revisit that pin? It holds back any other package that depends on dm-control.