Fetch repositories with --prune option by default.

This will allow us to not recreate branches that has been deleted on the upstream and the mirror.

CHANGELOG

+v4.0.3
+  - Fetch repositories with `--prune` option by default
 v4.0.2
   - Fix gitlab_custom_hook dependencies

lib/gitlab_projects.rb

@@ -205,6 +205,7 @@ class GitlabProjects
     $logger.info "Fetching remote #{@name} for project #{@project_name}."
     cmd = %W(git --git-dir=#{full_path} fetch #{@name})
+    cmd << '--prune'
     cmd << '--force' if forced
     cmd << tags_option
     pid = Process.spawn(*cmd)