Represent-boundaries

* Add `start_date` and `end_date` to boundaries. [31](https://github.com/opennorth/represent-boundaries/pull/31) (mileswwatkins)
* Increase length of `external_id` to 255 characters. [32](https://github.com/opennorth/represent-boundaries/pull/32) (evz)
* Add `blank=True` to `JSONField` for Django 1.9. [33](https://github.com/opennorth/represent-boundaries/pull/33) (jamesturk)

* Fix definition file loader to use an importer instead of `eval` [30](https://github.com/opennorth/represent-boundaries/pull/30) (paultag)
* If you were using `re` or other modules imported by `boundaries/__init__.py` in your definition files without importing them in your definition files, you must now import them in your definition files.

* Relax `jsonfield` version requirements.
* Fix assignment of default `srs` in `Feature` class, which was breaking Heroku static assets.

* Fix regression in slugless definition files.

* Eliminate Django 1.7 deprecation warning.

* Set default value on `jsonfield` fields.