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
OK, it worked for me.  Here's my output, including me fumbling with git pull:

https://gist.github.com/jwhitlock/a9dc65747211b57469ab

Some possibilities:
- Are you running the fix_heroku_50 branch?  There are code changes as well
- The "Cannot determine PostGIS version" also happens on database connection issues.  Did you mistype a password?
- Did you add the password to `DATABASE_URL` (last command, step 4)?
- Did you push the fix_heroku_50 branch, like `git push heroku fix_heroku_50:master ?