In some cases we've seen errors in Travis saying that it fails to
download the toolchain due to a bad signature. To prevent that from
happening we're only downloading using standard http instead of https.

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