Retire Travis CI builds

Since almost all Arctic Ice Studio projects already completely moved to
Circle CI (1) as single CI provider Nord will now also retire the
Travis CI (2) builds. It is not necessary to run both Circle CI and
Travis CI at the same time since the Circle CI API version 2.x (3) is by
far more advanced and provides all features of Travis CI.

References:
  (1) https://travis-ci.org
  (2) https://circleci.com
  (3) https://circleci.com/docs

GH-112