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>

.travis.yml

@@ -25,7 +25,7 @@ cache:
 
 install:
  # install latest device tree compiler
- - git clone --depth=1 https://git.kernel.org/pub/scm/utils/dtc/dtc.git /tmp/dtc
+ - git clone --depth=1 git://git.kernel.org/pub/scm/utils/dtc/dtc.git /tmp/dtc
  - make -j4 -C /tmp/dtc
  # prepare buildman environment
  - export BUILDMAN_ROOT="root:"