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>

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