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For some reason heroku auth:token was returning a wrong token for me, even after making sure that I was logged in to heroku on the command line.
After trying all the solutions posted, what worked for me was:
Go to Heroku Account Manually copy the API Key and then paste it into command line:
For the ones hosted at travis-ci.org: travis encrypt pasteAPIKeyHere --add deploy.api_key --org travis encrypt $(heroku auth:token) --add deploy.api_key --org