net/rubygem-gitaly: Update from 0.6.0 to 0.8.0

Changelog: https://gitlab.com/gitlab-org/gitaly/blob/v0.8.0/CHANGELOG.md

Add missing dependency devel/gitaly and adjust required version of
rubygem-google-protobuf. Older versions have a bug which make it
unusable with www/gitlab.

Reviewed by: Matthias Fechner <idefix@fechner.net>