TravisCI: run syntax checking after each change

[php-coder]

It would be great to checking module for syntax errors after each change. To do this we should:

1. create tests playbook (tests/test.yml)
2. add and configure .travis.yml for running command like ansible-playbook --syntax-check tests/test.yml --list-tasks
3. add badge to README.md

@see https://servercheck.in/blog/testing-ansible-roles-travis-ci-github