.travis.yml: only tag latest images on master

Ensure that only images built from the master branch get tagged with
`latest`.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

.travis.yml

@@ -28,4 +28,4 @@ script:
 after_success:
   - docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD"
   - docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
-  - make manifest && [ -z "$TRAVIS_TAG" ] && make manifest-latest
+  - make manifest && [ -z "$TRAVIS_TAG" ] && [ "$TRAVIS_BRANCH" == "master" ] && make manifest-latest