Upgrade Travis CI docker version to fix Molecule tests.

Author: geerlingguy

Diff for: .travis.yml

@@ -14,6 +14,10 @@ env:
     - MOLECULE_DISTRO: debian10
       MOLECULE_PLAYBOOK: playbook-calico.yml
 
+before_install:
+  # Upgrade Docker to work with docker-py.
+  - curl https://gist.githubusercontent.com/geerlingguy/ce883ad4aec6a5f1187ef93bd338511e/raw/36612d28981d92863f839c5aefe5b7dd7193d6c6/travis-ci-docker-upgrade.sh | sudo bash
+
 install:
   # Install test dependencies.
   - pip install molecule yamllint ansible-lint docker