Explicitly start docker service
Explicitly start docker service

so that (hopefully) the travis-worker service can start, too

travis-ci/travis-cookbooks,ljharb/travis-cookbooks,spurti-chopra/travis-cookbooks,travis-ci/travis-cookbooks,travis-ci/travis-cookbooks,spurti-chopra/travis-cookbooks,ljharb/travis-cookbooks,ljharb/travis-cookbooks,spurti-chopra/travis-cookbooks