Bump version to 4.0.0.
Bump version to 4.0.0.

Breaking Changes
  * #79 "django.contrib.contenttypes" is now included in library's base
    settings. If copying these over, you'll need to deduplicate.

New Features
  * #79 Allow search handler to be defined via `SEARCH_HANDLER` setting
  * #83 Allow search results page size to be requested
  * #82 Add Postgres backend for search. See README for details.

Misc
  * #77 Allow ALLOWED_HOSTS to be set via env vars
  * #78 Use webargs library for checking search input
  * #80 Test different backends in different tox envs
  * #81 Remove QuantifiedCode badge

eregs/regulations-core,18F/regulations-core