Hotfix a bug caused by me force commiting to develop branch (#2004)

Author: wangkuiyi

.travis.yml

@@ -28,7 +28,6 @@ jobs:
       name: "Pre-commit Check"
       script:
         - set -e
-        - export DOCKER_BUILDKIT=1
         - docker build --target dev -t elasticdl:dev -f
           elasticdl/docker/Dockerfile .
         - docker run --rm -it -v $PWD:/work -w /work
@@ -37,7 +36,6 @@ jobs:
       name: "Tests"
       script:
         - set -e
-        - export DOCKER_BUILDKIT=1
         # Set up Kubernetes environment
         - bash scripts/setup_k8s_env.sh
         - bash elasticdl/docker/build_all.sh