Fix Docker install on TravisCi by allowing the docker-ce package to be downgraded

[dhui]

• Started happening since TravisCI updated docker-ce https://docs.travis-ci.com/user/build-environment-updates/2017-12-12/
• Can't use v17.09 since the git tag doesn't exist yet

[dhui]

Fixed in: https://github.com/golang-migrate/migrate/commit/303cd74e5f037fe78847127066e8e055031440d0