Django-publications-bootstrap

Changed
- Relax dependency on django-echoices at specific version

Removed
- Support of Django 1.9 (deprecated since April 2017)

- Django 2.0 support

- Avoid RemovedInDjango20Warnings while testing
- Support of Django 1.11

- Fix [25](https://github.com/mbourqui/django-publications-bootstrap/issues/25)
- Fix [26](https://github.com/mbourqui/django-publications-bootstrap/issues/26)
- Update tests to better cover citekey generation
- Add tests for Publication.month property
- Set warnings for future refacto of Publication methods to properties

- Fix [23](https://github.com/mbourqui/django-publications-bootstrap/issues/23)

- Refacto models
* Renamed `Custom[Link|File]` to `Publication[Link|File]`
* Replace Enums with django-echoices
- Drop self-hosted libraries for PyPI dependencies
- Drop support of Python 2.7
- Drop support of Django 1.8 due to django-echoices