travis: Add initial travis configuration

All supported projects in build.git besides DRA7xx will be fully built
when Travis is enabled. This configuration will try to download and
cache the toolchains and ccache (to be verified).

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by : Jerome Forissier <jerome.forissier@linaro.org>