Add back dist: xenial

Until we will be sure about system python detection in tox-travis
or we find another way of how to enforce running molecule on
system python, we must prevent the bumping system python version
when Travis team bumps default building environmet by setting the
building environment explicitly.

.travis.yml

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: MIT
 ---
+dist: xenial
 language: python
 matrix:
   include: