use bionic for host machine

.travis.yml

@@ -2,10 +2,8 @@
 # https://github.com/ros-infrastructure/ros_buildfarm/blob/master/doc/jobs/prerelease_jobs.rst
 # while this doesn't require sudo we don't want to run within a Docker container
 sudo: true
-dist: trusty
+dist: bionic
 language: python
-python:
-  - "3.4"
 env:
   global:
     - JOB_PATH=/tmp/devel_job