.travis.yml: only tag latest image if not git tag

If we tag a release for, e.g. 0.1.1, after we've already cut a 0.2.0
tag, then CI would tag the 0.1.1 image as `latest`, which is confusing.
This commit ensures that we only tag the `latest` image when building
from master.

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 && make manifest-latest
+  - make manifest && [ -z "$TRAVIS_TAG" ] && make manifest-latest