Build with bazel 0.27 fails

The build fails due to protobuf:

```
DEBUG: Call stack for the definition of repository 'com_google_protobuf' which is a git_repository (rule definition at /build/output/external/bazel_tools/tools/build_defs/repo/git.bzl:252:18):
- /build/source/WORKSPACE:5:1
ERROR: /build/output/external/com_google_protobuf/protobuf.bzl:130:19: Traceback (most recent call last):
File "/build/output/external/com_google_protobuf/protobuf.bzl", line 125
rule(attrs = {"srcs": attr.label_list...()}, <2 more arguments>)
File "/build/output/external/com_google_protobuf/protobuf.bzl", line 130, in rule
attr.label(cfg = "host", executable = True, sin..., ...)
'single_file' is no longer supported. use allow_single_file instead. You can use --incompatible_disable_deprecated_attr_params=false to temporarily disable this check.
```

This will be fixed upstream by https://github.com/protocolbuffers/protobuf/pull/6304, so sonnets protobuf version should be updated after that is merged.