Travis CI: remove Python 3.4 and add Python 3.6
Travis CI: remove Python 3.4 and add Python 3.6

Available Python versions are listed at:
https://docs.travis-ci.com/user/languages/python/#Choosing-Python-versions-to-test-against

LudovicRousseau/pyscard,LudovicRousseau/pyscard,LudovicRousseau/pyscard