In the Heroku documentation; Getting started whit rails 4, they say:
You will also need to remove the username field in your database.yml if there is one so: In file config/database.yml remove: username: myapp
Then you just delete that line in "development:", if you don't pg tells to the database that works under role "myapp"
This line tells rails that the database myapp_development should be run under a role of myapp. Since you likely don’t have this role in your database we will remove it. With the line remove Rails will try to access the database as user who is currently logged into the computer.
Also remember to create the database for development:
$createdb myapp_development
Repleace "myapp" for your app name