Always regenerate ruby, not just on release

The split into multiple .proto files forces us to have an auto-generated ruby/lib/gitaly.rb, so that clients only need to require 'gitaly'.

We previously wrote some code that generates ruby/lib/gitaly.rb but it only ran on release. This change pushes the generator into the normal generate-script so that unreleased feature branches also benefit from an up-to-date ruby/lib/gitaly.rb.