Upgrade CI to concourse-release-scripts 0.3.4

Author: bclozel

ci/tasks/promote-version.yml

@@ -4,7 +4,7 @@ image_resource:
   type: registry-image
   source:
     repository: springio/concourse-release-scripts
-    tag: '0.3.4-SNAPSHOT'
+    tag: '0.3.4'
 inputs:
   - name: git-repo
   - name: artifactory-repo