docker: update Travis docker image

This is still poorly documented by Travis but according to:

  https://docs.travis-ci.com/user/common-build-problems/#Running-a-Container-Based-Docker-Image-Locally

their reference images are now hosted on Docker Hub. So we update the
FROM line to refer to the new default image. We also need a few
additional tweaks:

  - re-enable deb-src lines for our build-dep install
  - add explicit PATH definition for tools
  - force the build USER to be Travis
  - add clang to FEATURES for our test-clang machinery

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/dockerfiles/travis.docker

@@ -1,8 +1,13 @@
-FROM quay.io/travisci/travis-ruby
+FROM travisci/ci-garnet:packer-1512502276-986baf0
 ENV DEBIAN_FRONTEND noninteractive
 ENV LANG en_US.UTF-8
 ENV LC_ALL en_US.UTF-8
+RUN cat /etc/apt/sources.list | sed "s/# deb-src/deb-src/" >> /etc/apt/sources.list
 RUN apt-get update
 RUN apt-get -y build-dep qemu
 RUN apt-get -y install device-tree-compiler python2.7 python-yaml dh-autoreconf gdb strace lsof net-tools
-ENV FEATURES pyyaml
+# Travis tools require PhantomJS / Neo4j / Maven accessible
+# in their PATH (QEMU build won't access them).
+ENV PATH /usr/local/phantomjs/bin:/usr/local/phantomjs:/usr/local/neo4j-3.2.7/bin:/usr/local/maven-3.5.2/bin:/usr/local/cmake-3.9.2/bin:/usr/local/clang-5.0.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+ENV FEATURES clang pyyaml
+USER travis