Fix post-receive feature flag not working in gitaly-ruby

Even though the feature flag to use the Go implementation of the post-receive hook is on by default, gitaly-ruby was still using the Ruby version. This was happening because gitaly-ruby did not activate the feature flag because the go-postreceive-hook flag didn't start with the prefix gitaly-feature-ruby-. We now allow all feature flags that end with -hook values to be used within gitaly-ruby.

Relates to gitlab#258833 (closed)