Fix heroku login


If you are using this awesome Heroku accounts plugin: https://github.com/ddollar/heroku-accounts, you might not be able to login using the heroku toolbelt after an api key reset. The reason is because the plugin stores a copy of the key in:

~/.heroku/accounts/<account_name>

So to be able to log in again, you just need to update this file with the new api key. Then a simple heroku login should work.