travis: use tee while pulling docker images

Shows us progress while downloading images and we can verify that the
images match what we expect. `tee` is mainly used to disable the pull
animation that was spamming the logs otherwise.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

.travis.yml

@@ -13,7 +13,7 @@ env:
 
 # Trusty (aka 14.04) is way way too old, so run in docker...
 script:
-  - docker pull cdecker/lightning-ci:${ARCH}bit > /dev/null
+  - docker pull cdecker/lightning-ci:${ARCH}bit | tee
   - docker run --rm=true -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make -j3 DEVELOPER=${DEVELOPER}
   - docker run --rm=true -e NO_VALGRIND=${NO_VALGRIND:-0} -e TEST_DEBUG=${TEST_DEBUG:-0} -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make check DEVELOPER=${DEVELOPER}
   - docker run --rm=true -v "${TRAVIS_BUILD_DIR}":/build -t cdecker/lightning-ci:${ARCH}bit make check-source