Upgrade docker-compose inside travis (fix build errors)

.travis.yml

@@ -4,6 +4,7 @@ addons:
   apt:
     packages:
       - docker-ce
+
 env:
   - MAILU_VERSION=$TRAVIS_BRANCH
 language: python
@@ -11,8 +12,11 @@ python:
   - "3.6"
 install:
   - pip install -r tests/requirements.txt
+  - sudo curl -L https://github.com/docker/compose/releases/download/1.23.0-rc3/docker-compose-`uname -s`-`uname -m` -o /usr/local/bin/docker-compose
+  - sudo chmod +x /usr/local/bin/docker-compose
 
 before_script:
+  - docker-compose -v
   - docker-compose -f tests/build.yml build
   
 script:
@@ -30,4 +34,3 @@ deploy:
   on:
     all_branches: true
     condition: -n $DOCKER_UN
-