Builds on TravisCI are failing due to 404 erros

[fperetti]

It seems that some python images are missing:

`Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.3.tar.bz2

0.14s$ curl -sSf -o python-3.3.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404 Not Found
Unable to download 3.3 archive. The archive may not exist. Please consider a different version.`

e.g: https://travis-ci.org/jarus/flask-testing/builds/575623697?utm_source=github_status&utm_medium=notification