Deployment fails for 'nightly' Python builds

[jakubka]

Our Travis build is configured to run in Python 3.5 and 'nightly' Python: https://github.com/Skyscanner/pyfailsafe/blob/master/.travis.yml#L4

The problem is that it tries to deploy to pypi from both of these environments. The first one succeeds, but second one always fails: https://travis-ci.org/Skyscanner/pyfailsafe/jobs/408569147

(most likely because the package is already published).

The desired behaviour is that tests should be run in all environments (3.5, 3.6, 3.7, nightly) and after all these tests succeed, the deployment should be run once.