tests/docker: update Travis image to a more current version

This isn't the latest one available on hub.docker.com but it does
match the ID reported by the Xenial builds running on Travis:

  instance: ... travis-ci-sardonyx-xenial-1553530528-f909ac5
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

tests/docker/dockerfiles/travis.docker

-FROM travisci/ci-sardonyx:packer-1546978056-2c98a19
+#
+# Travis Image - this is broadly the same image that we run our CI
+# tests on.
+#
+FROM travisci/ci-sardonyx:packer-1552557266-f909ac5
 ENV DEBIAN_FRONTEND noninteractive
 ENV LANG en_US.UTF-8
 ENV LC_ALL en_US.UTF-8