Bump protobuf from 3.9.1 to 3.15.0 in /5_visionbot/src

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.9.1 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@v3.9.1...v3.15.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

5_visionbot/src/requirements.txt

@@ -16,7 +16,7 @@ Markdown==3.1.1
 MarkupSafe==1.1.1
 numpy==1.17.0
 Pillow==6.1.0
-protobuf==3.9.1
+protobuf==3.15.0
 PyYAML==5.1.2
 scipy==1.3.0
 six==1.12.0