Udapte .proto files to their latest version

Noteworthy changes include:
* From remote_execution.proto:
  - New message field in ExecuteResponse [1].
  - Output file and dir. parent folders clarifications [2].
  - Better doc. for do_not_cache/skip_cache_lookup [3].
  - Documention updates.
* From semver.proto:
  - Documentation updates.

[1] https://github.com/bazelbuild/remote-apis/pull/36
[2] https://github.com/bazelbuild/remote-apis/pull/42
[3] https://github.com/bazelbuild/remote-apis/pull/55