feat(CI): Enforce strict checking of YAML files

Use the `--strict` flag to check all rules on local YAML files, to
prevent all problems (including warnings). This includes the newly added
`truthy` rule.

.travis.yml

@@ -11,7 +11,7 @@ install:
   - pip install .
 script:
   - flake8 .
-  - yamllint $(git ls-files '*.yaml' '*.yml')
+  - yamllint --strict $(git ls-files '*.yaml' '*.yml')
   - coverage run --source=yamllint setup.py test
 after_success:
   coveralls