Makefile: Fix dependencies when generating Ruby Protobuf code

In order to generate the Protobuf code for Ruby, we have a Makefile target that executes our own generate-proto-ruby script directly. As a result, the script doesn't use fixed dependencies specified in our Gemfile and may thus use whatever latest dependencies it finds to generate the code. This breaks as soon as there's a different version of the google-protobuf Gem installed on the developer's machine, as formats have frequently been changing across versions.

Fix the issue by using bundle exec, pointing to our Gemfile. This causes us to always use the specified dependency versions and hopefully make code generation more deterministic.