Protobuf version compatibility with Bazel

[zaqqwerty]

Downstream projects that use Bazel versions 0.22 or greater are only compatible with protobuf versions after 3.6.1.2. This is due to use of a constant that the newer versions of Bazel consider deprecated (for details, see discussion at bazelbuild/bazel#5827). So I propose updating to protobuf version 3.7.0, the first minor version of protobuf with correct compatibility.