Remove old docker.list to avoid getting unstable Docker version

.travis.yml

@@ -7,7 +7,8 @@ env:
   - TEST_TARGET: test-alpine
 
 before_install:
-  - sudo apt-get remove docker docker-engine
+  - sudo apt-get -y remove docker docker-engine docker-ce
+  - sudo rm /etc/apt/sources.list.d/docker.list
   - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
   - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
   - sudo apt-get update