travis: update python versions

3.4 is no longer available in bionic-based travis. Use this occasion to
add 3.6 and 3.7.

QubesOS/qubes-issues#4613

.travis.yml

@@ -22,11 +22,13 @@ jobs:
    - env: TESTS_ONLY=1
      python: 2.7
    - env: TESTS_ONLY=1
-     python: 3.4
+     python: 3.5
    - env: TESTS_ONLY=1
-     python: 3.5
+     python: 3.6
+   - env: TESTS_ONLY=1
+     python: 3.7
    - stage: deploy
-     python: 3.5
+     python: 3.6
      env: DIST_DOM0=fc29 TESTS_ONLY=
      script: ~/qubes-builder/scripts/travis-deploy