Pin Travis-ci build environment to previous default: Trusty

Travis-ci changed the default build environment to Xenial as explained in https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment
This causes builds meant for Debian Jessie to break as noted by @wRAR in #3917 (comment)

This change pins the environment to known working ubuntu trusty distribution prior to dropping Jessie support and upgrade to Xenial as base.

Closes #1369

.travis.yml

@@ -1,4 +1,5 @@
 language: python
+dist: trusty
 branches:
   only:
     - master