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

(cherry picked from commit 9f0dc7dc34)

.travis.yml

@@ -22,10 +22,12 @@ jobs:
   include:
    - env: TESTS_ONLY=1
      python: 2.7
-   - env: TESTS_ONLY=1
-     python: 3.4
    - env: TESTS_ONLY=1
      python: 3.5
+   - env: TESTS_ONLY=1
+     python: 3.6
+   - env: TESTS_ONLY=1
+     python: 3.7
    - stage: deploy
      python: 3.5
      env: DIST_DOM0=fc25 TESTS_ONLY=