Fix PostGIS support in new-django-app template

[jeancochrane]

Overview

Update the Dockerfile and settings.py files in the new-django-app template to properly install GDAL and set the Django engine to support PostGIS as a backend when postgis is True.

Handles #93.

Testing Instructions

• Change into the docker/templates/ directory
• Run docker-compose run --rm cookiecutter -f new-django-app
• Accept all default settings, but set postgis to True
• Change into my-new-app/ and run docker-compose up --build
• Confirm that the app builds properly and is visible on http://localhost:8000
• Shut down the app with docker-compose down --volumes
• Change back to the docker/templates/ directory and remove the new app with rm -RF my-new-app
• Remove old image with docker rmi my-new-app
• Create a new app without PostGIS support by running docker-compose run --rm cookiecutter -f new-django-app again
• Confirm that the Dockerfile and settings.py files look syntactically correct