travis: drop python 3.4, add python 3.6 and 3.7
travis: drop python 3.4, add python 3.6 and 3.7

Python 3.4 is EOL, and not supported by recent lxml already.
This also means dropping jessie tests.

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client