Merge pull request #296 from integer32llc/travis-docker

Update Docker login procedure to newest recommended by Travis

Author: shepmaster

.travis/build-containers.sh

@@ -7,7 +7,7 @@ if [[ ("${TRAVIS_PULL_REQUEST}" == "false") &&
           (-n "${DOCKER_USERNAME}") &&
           (-n "${DOCKER_PASSWORD}") ]]
 then
-    docker login -u="${DOCKER_USERNAME}" -p="${DOCKER_PASSWORD}"
+    echo "${DOCKER_PASSWORD}" | docker login -u "{$DOCKER_USERNAME}" --password-stdin
     export PERFORM_PUSH="true"
     # Which images to build are set via .travis.yml
     cd compiler && ./build.sh