Use event ref

Author: Bastien Abadie

Diff for: .taskcluster.yml

@@ -25,8 +25,8 @@ tasks:
       else: ${event.repository.html_url}
 
     version:
-      $if: 'head_branch[:10] == "refs/tags/"'
-      then: {$eval: 'head_branch[10:]'}
+      $if: 'tasks_for == "github-push" && head_branch[:10] == "refs/tags/"'
+      then: {$eval: 'event.ref[10:]' }
       else: "latest"
 
   in: