Include GitLab CI.

The versions previously used (Trusty) doesn't support newer versions of
Python. Besides, I should have updated anyway.

It's not supported anymore and PyYAML refuses to install with that
version.

- Update Ansible.
- More pre-commit checks.
- Replace the Vault shell script with a simpler grep hook.

- Update to the latest version of Ansible.
- Update to local pre-commit checks.

- Raised requirements to Python 3.4 or newer.
- Read version in setup.py from the VERSION file.

- Added flake8 test for setup.py.

- Removed useless pre-commit checks.
- Added Travis CI test, badge to README.
- Added pre-commit test for outdated requirements.

- Renamed ansible-pre-commit to ansible-syntax-check in preperation for adding another hook, updated hooks.yml and README accordingly.

- Bumped patch version.

- Bump minor version.