Automatic deploy to Heroku is not working

[willu47]

Travis CI is setup to deploy to Heroku automatically upon a tag pushed to the master branch, but the deploy is failing due to a constraint at the Heroku end. The error

API request failed.
Message: Your account has reached its concurrent builds limit
Reference: 
failed to deploy

Apparently, to remove this build limit of one, you need to verify the Heroku account: https://devcenter.heroku.com/articles/account-verification