Fix Heroku setup script

The DEVISE env var was misnamed, and the api endpoint was misnamed in the usage instructions.

I also removed the call to script/users to allow people to create their own users from scratch as opposed to pre-populating their production DB with dummy data.

The script now enforces the presence of each option and argument.
Update README with additional Heroku setup instructions
Update header for Heroku deployment in README
Merge pull request #55 from codeforamerica/update-heroku-script-38

Update heroku script. Fixes #38