Makefile: Allow building the Gitaly gem without a tag

The script to build the Gitaly ruby gem compares the VERSION file with the current tag (via git describe --tags), this is a safety check so that we don't build an publish non-tagged releases.

But this also blocks us from building the gem for local testing. So add a new option --version-check to the build script and call this option only when make publish-proto-gem is used, so that we retain this safety check when trying to publish a gem. For make build-proto-gem make this an option with the default behavior allowing to build gems without checking the version.