Fix CI push behaviour

CI was using a test of ! -s which appeared to give a false positive
and try to push when it shouldn't. I changed it to -z which checks
for an empty string instead.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

.travis.yml

@@ -20,7 +20,7 @@ script:
     - sh contrib/ci.sh
 
 after_success:
-    - if [ ! -s "$TRAVIS_TAG" ] ; then 
+    - if [ ! -z "$TRAVIS_TAG" ] ; then 
         docker tag $DOCKER_NS/gateway:latest-dev $DOCKER_NS/gateway:$TRAVIS_TAG;
         docker login -u=$DOCKER_USERNAME -p=$DOCKER_PASSWORD;
         docker push $DOCKER_NS/gateway:$TRAVIS_TAG;