Travis: use default arm-linux-gnueabihf- compiler

The download of the Linaro compiler as well as the installation of
its dependencies (including some 32-bit libraries) is replaced by
the installation of the default gcc-arm-linux-gnueabihf package.

This speed up the build time by about a minute.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>