I think this is what you need: https://github.com/pyslackers/slack-sansio/blob/master/.travis.yml#L37
I didn't test it myself but this is the way it should be implemented, in my opinion.
For anyone stumbling upon this issue thread, the above link is not longer available. The file can now be found at https://github.com/pyslackers/slack-sansio/blob/2adb7c285deccbbb915d3890c27dc6d9adf38a3c/.travis.yml
Most helpful comment
I think this is what you need: https://github.com/pyslackers/slack-sansio/blob/master/.travis.yml#L37
I didn't test it myself but this is the way it should be implemented, in my opinion.