Update name of heroku free postgres add on

[phalgun]

The mentioned 'shared-database' addon is currently not available in heroku and is replaced by 'heroku-postgresql:dev'. Check https://postgres.heroku.com/blog/past/2012/4/26/heroku_postgres_development_plan/

[coveralls]

Coverage remained the same when pulling 781dfac on phalgun:patch-1 into 215393a on openhatch:master.

[paulproteus]

Hi @phalgun !

Pardon the delay. We took a look at this during the sprint, and @natea wrote up a new version of the document, and tested it. It turns out that Heroku seems to detect the need for a database, so we've removed this discussion anyway.

I'm closing this for now; I want to say thank you! Your pull request here prompted a much broader analysis of the app on Heroku, and now the database migrations run properly on Postgres as well!