CI: Use the correct bash syntax

345afee677 broke the bash
extreppsion that evaluates the strings, causing images to
never be pushed.

.gitlab-ci.yml

@@ -50,7 +50,7 @@ test manifest:
 
     # Push the images to the upstream registry
     - |
-      if [ "$CI_COMMIT_REF_NAME" = "master" && "$CI_PROJECT_NAMESPACE" = "gstreamer" ]; then
+      if [ "$CI_COMMIT_REF_NAME" = "master" -a "$CI_PROJECT_NAMESPACE" = "gstreamer" ]; then
         docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
         docker push ${REF}
         docker push ${LATEST}