Travis: Remove unsupported python versions

They're not really telling us anything useful.

.travis.yml

@@ -105,12 +105,6 @@ matrix:
     ## End of Life: 1 January 2020
     ## https://www.python.org/dev/peps/pep-0373/#id2
     - python: "2.7"
-    ## Final Release: 17 March 2019
-    ## https://www.python.org/dev/peps/pep-0429/#release-schedule
-    - python: "3.4"
-    ## Final Release: 17 March 2019
-    ## https://www.python.org/dev/peps/pep-0478/#id4
-    - python: "3.5"
     ## End of Life: December 2021
     ## https://www.python.org/dev/peps/pep-0494/#lifespan
     - python: "3.6"