travis: replace Trusty-based LLVM toolchain apt-get with apt addon

Trusty's LLVM toochain repository was whitelisted some time ago. See:
https://github.com/travis-ci/apt-source-whitelist/commit/479067c5e74cb0c1e2419209179b1afe2edce274

Signed-off-by: Andres Gomez <agomez@igalia.com>
[Emil Velikov]
 - set sudo to false
 - reference the Trusty change (Rhys)
 - keep libedit-dev
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed