Make sure docker driver is installed with molecule.

Author: geerlingguy

.travis.yml

@@ -20,7 +20,7 @@ before_install:
 
 install:
   # Install test dependencies.
-  - pip install molecule yamllint ansible-lint docker
+  - pip install molecule[docker] yamllint ansible-lint docker
 
 before_script:
   # Use actual Ansible Galaxy role name for the project directory.