Fix python 3 version used by travis (#1016)
Fix python 3 version used by travis (#1016)

Travis now uses
[xenial](https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment)
as the default build environment. The CI uses system site packages, but
this option only works with the default python 3 version. In xenial this
version is [python 3.5](https://packages.ubuntu.com/xenial/python3).

Signed-off-by: Shane Loretz<7fde345e8e2e22c78a1570a4807e4629866dc7de@openrobotics.org>
ros/catkin,ros/catkin,ros/catkin