python-django: security bump to version 1.11.15

python-django: security bump to version 1.11.15

Bump to the latest release of the 1.11.x LTS series as 1.10.x is no longer
supported upstream:

https://www.djangoproject.com/download/

Fixes the following security issues:

- CVE-2017-12794: Possible XSS in traceback section of technical 500 debug
  page (1.11.5)

- CVE-2018-6188: Information leakage in AuthenticationForm (1.11.10)

- CVE-2018-7536: Denial-of-service possibility in urlize and urlizetrunc
  template filters (1.11.11)

- CVE-2018-7537: Denial-of-service possibility in truncatechars_html and
  truncatewords_html template filters (1.11.11)

- CVE-2018-14574: Open redirect possibility in CommonMiddleware (1.11.15)

Also add a hash for the license file.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>