text: Title: Can't set CONN_MAX_AGE to None
  Body: When setting DATABASE_URL to something like `postgres://posgres@db:5432/postgres?conn_max_age=None` the end result is:
  
  ``` python
  {
      'ENGINE': 'django.db.backends.postgresql_psycopg2',
      'HOST': 'db'
      'PORT': 5432,
      'USER': 'postgres',
      'PASSWORD': None,
      'NAME': 'postgres',
      'CONN_MAX_AGE': 'None',
      'OPTIONS': {},
  }
  ```
  
  Where CONN_MAX_AGE has an invalid value: it has to be set to either a float or None, but django-environ returns a str.
  