Use proper environment name for docker username

travis/travis-script.bash

@@ -6,6 +6,6 @@ VERSION=$(git rev-parse --short HEAD)
 docker build -t jupyter/repo2docker:${VERSION} .
 
 if [[ "$TRAVIS_PULL_REQUEST" == "false" ]]; then
-    docker login -u ${DOCKER_USERNAME} -p "${DOCKER_PASSWORD}"
+    docker login -u ${DOCKER_LOGIN} -p "${DOCKER_PASSWORD}"
     docker push jupyter/rep2docker:${VERSION}
 fi