Relax the semver requirement for protobuf

[clintfred]

This will allow downstream consumers to choose their version of protobuf without causing build issues.

I realized this while looking at https:/tikv/rust-prometheus/blob/master/Cargo.toml

I tested this by git reseting logdriver to an old revision that used protobuf 2.14. It compiled file. It also compiled fine with a dependency of 2.17.

There might still be problems in upgrading to newer version because we have custom build.rs logic for the proto generation, but at least cargo will be able to successfully select a version.

If we merge this, I think we can close #188

[giarc3]

My opinion is that ^ beats ~ for any dependency that we trust to conform to semver.
Your change is equivalent to ^2, which might be a little more clear? But maybe not.

[coltfred]

Seems entirely reasonable to me.