Travis: do not use --use-mirrors  (rebased onto dev_4_4)



This is the same as gh-2061 but rebased onto dev_4_4.

----

This PR should fix the recent failing Travis builds. The latter seemed to be related to our using of `pip install … --use-mirrors` in .travis.yml.

See also https://github.com/travis-ci/docs-travis-ci-com/pull/10