Enable to use newer protobuf

[dskkato]

• remove protobuf version restriction
• fix CI

Closes#414

[dskkato]

@adamcrume Could you check this PR if you have time?

[adamcrume]

The problem is that the generated code (which is checked in) is only compatible with the exact same version of protobuf that generated it. This is a very annoying property of the protobuf crate, but I can't change that.

We could switch to generating the code in a build script, which means that we need the proto files available at build time. We could do that by adding TensorFlow as a git submodule, but it's huge (hundreds of megabytes), and apparently cargo doesn't work well with submodules (rust-lang/cargo#7987). Possibly a better way is to just manually (with a script) copy the protos from TensorFlow into this crate for each version upgrade.

[dskkato]

hmmm
https://github.com/tensorflow/rust/blob/master/src/protos/allocation_description.rs#L1-L1