chore: add docker credentials to travis

Author: twilio-dx

.travis.yml

@@ -18,6 +18,7 @@ before_script:
 - chmod +x ./cc-test-reporter
 - "./cc-test-reporter before-build"
 script:
+- echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
 - make test-docker
 after_script:
 - make test-install