Bump protobuf from 3.11.3 to 3.15.0 in /updater

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.11.3 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.11.3...v3.15.0)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

updater/requirements.txt

@@ -10,7 +10,7 @@ googleapis-common-protos==1.51.0
 httplib2==0.17.0
 idna==2.9
 oauthlib==3.1.0
-protobuf==3.11.3
+protobuf==3.15.0
 pyasn1==0.4.8
 pyasn1-modules==0.2.8
 pytz==2019.3