travis: Support LLVM 3.8+ on Trusty-based Travis-CI via apt-get not apt addon

travis: Support LLVM 3.8+ on Trusty-based Travis-CI via apt-get not apt addon

Per comments by Travis-CI, the apt addon is only really needed for the
container-based Precise builds, as they don't yet support Trusty on that platform.

Mesa currently uses Trusty fully-virtualized environment (due to sudo: required).

See further:
https://docs.travis-ci.com/user/trusty-ci-environment/#Fully-virtualized-via-sudo%3A-required
https://github.com/travis-ci/apt-source-whitelist/pull/205#issuecomment-216054237

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>