Make deadlinks a deploy stage, so gated by the initial build

.gitlab-ci.yml

@@ -6,8 +6,9 @@ test:
   script:
     - bundle exec rake test
 
+# Not a test stage to prevent race conditions with gem installations.
 deadlinks:
-  stage: test
+  stage: deploy
   script:
     - bundle exec rake testlinks
   allow_failure: true