travis: force llvm-3.3 for "make Gallium ST Other"

travis: force llvm-3.3 for "make Gallium ST Other"

In Ubuntu Trusty the default version of llvm is 3.4 and the build was
actually randomly picking 3.5 or 3.9. Adding libunwind would then result
is build success or failure depending of what version was picked.

Install the llvm-3.3-dev package and force its use: On one hand it is
the minimum required version we want to the build test against, and on
the other hand forcing the version stabilizes the build.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>