Revert django requirement to 1.9.7
Revert django requirement to 1.9.7

This line meant that if we install the library after installing django
1.9.7, we then uninstall and upgrade django:

```
Step 6/9 : RUN pip install git+https://nl-bot:2ec91393d121a10a83785c8df77e83ee7c417220@github.com/networklocum/django-changes-on-save@master
 ---> Running in 5b0e34e4b503
Collecting git+https://nl-bot:****@github.com/networklocum/django-changes-on-save@master
  Cloning https://nl-bot:****@github.com/networklocum/django-changes-on-save (to revision master) to /tmp/pip-req-build-3awe8mqs
  Running command git clone -q 'https://nl-bot:****@github.com/networklocum/django-changes-on-save' /tmp/pip-req-build-3awe8mqs
Requirement already satisfied (use --upgrade to upgrade): django-changes-on-save==0.0.1 from git+https://nl-bot:****@github.com/networklocum/django-changes-on-save@master in /usr/local/lib/python3.6/site-packages
Requirement already satisfied: django-dirtyfields in /usr/local/lib/python3.6/site-packages (from django-changes-on-save==0.0.1) (1.3.1)
Collecting django>=1.11.20
  Downloading https://files.pythonhosted.org/packages/55/d1/8ade70e65fa157e1903fe4078305ca53b6819ab212d9fbbe5755afc8ea2e/Django-3.0.2-py3-none-any.whl (7.4MB)
Requirement already satisfied: pytz>=2015.7 in /usr/local/lib/python3.6/site-packages (from django-dirtyfields->django-changes-on-save==0.0.1) (2016.4)
Collecting sqlparse>=0.2.2
  Downloading https://files.pythonhosted.org/packages/ef/53/900f7d2a54557c6a37886585a91336520e5539e3ae2423ff1102daf4f3a7/sqlparse-0.3.0-py2.py3-none-any.whl
Collecting asgiref~=3.2
  Downloading https://files.pythonhosted.org/packages/a5/cb/5a235b605a9753ebcb2730c75e610fb51c8cab3f01230080a8229fa36adb/asgiref-3.2.3-py2.py3-none-any.whl
Building wheels for collected packages: django-changes-on-save
  Building wheel for django-changes-on-save (setup.py): started
  Building wheel for django-changes-on-save (setup.py): finished with status 'done'
  Created wheel for django-changes-on-save: filename=django_changes_on_save-0.0.1-cp36-none-any.whl size=6026 sha256=2b1af42cfcc3002fef15c044e7c83a3aab63124f119651d02de249e5118f8e1f
  Stored in directory: /tmp/pip-ephem-wheel-cache-jgc3aumo/wheels/3d/0f/dc/b32dc9fcdf7c43325ebd26ed928645a64a5f12be53fb5f3384
Successfully built django-changes-on-save
Installing collected packages: sqlparse, asgiref, django
  Found existing installation: Django 1.9.7
    Uninstalling Django-1.9.7:
      Successfully uninstalled Django-1.9.7
```
...which is causing problems

for some reason if you install django and the library in the same
command, it just ignores the django 1.11 requirement:

```
RUN pip install django==1.9.7 git+https://nl-bot:2ec91393d121a10a83785c8df77e83ee7c417220@github.com/networklocum/django-changes-on-save@master
```

...which is why it's worked fine so far

I don't think it makes sense for this library to be worrying about
security flaws in its dependencies anyway. That should be the
responsiblity of the end-consumer. This requirements.txt should just
specify the minimum requirements for the library itself to function.

networklocum/django-changes-on-save