Get heroku manual build working

- Remove "Deploy to Heroku" button, since no free option
- Uses https://github.com/cyberdelia/heroku-geo-buildpack.git
- If Heroku detected, database driver switched to postgis
- Retain staticfiles folder for working ./manage.py collectstatic
- app.json has the minimal config for DEBUG=False config, loads data
- Closes #50
But this is right, it gives me:
```
bpzgroovecoder=> SELECT PostGIS_Version();
            postgis_version
---------------------------------------
 2.1 USE_GEOS=1 USE_PROJ=1 USE_STATS=1
(1 row)
```
Probably because RDS created the database before PostGIS was installed?