* Add source code URI to the Ruby gemspec file

As a developer, I would like to easily find the source code origin for this gem when browsing through https://rubygems.org/gems/google-protobuf.
In order to solve this, I've followed instructions at https://guides.rubygems.org/specification-reference/#metadata by adding the source_code_uri metadata key pair.

* Use git tag instead of fixed "master" string

* Substitute the string, not the Gem::Version