CI: use python 3.6 (#393)

python 2.7 is dead
python 3.6 is supported until 2021-12
It would be better to move to python 3.8 right away,
but since it is not installed by default in Travis,
having to download it in each job would make the builds significantly slower.

.travis.yml

@@ -1,6 +1,8 @@
 language: python
 python:
-    - "2.7"
+    - "3.6"
+
+dist: bionic
 
 # Cache PlatformIO packages using Travis CI container-based infrastructure
 sudo: false