travis-ci: Use a git URI for dtc.git

Currently we fail to fetch the dtc.git tree due to an SSL issue within
the travis-ci environment.  The easiest fix here is to switch to a git
URI.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>