[CI] travis doesn't support xenial / bionic, disable python 3

cf. https://github.com/travis-ci/travis-ci/issues/5821

Python 3 is tested by gitlab anyways.

.travis.yml

 language: python
 python:
   - "2.7"
-  - "3.6"
 sudo: required
-dist: bionic
+dist: trusty
 compiler:
   - gcc
 # - clang