Travis: use default arm32 compiler; fetch minimal dependencies

- Install the arm32 compiler provided by the distribution instead
of the Linaro compiler
- Install the minimal set of dependencies required by the Linaro
Aarch64 compiler

This speeds up the build significantly.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>