Switch Travis testing to ubuntu 18.04

.travis.yml

@@ -7,20 +7,20 @@ matrix:
   allow_failures:
     - julia: nightly
 sudo: required
-dist: xenial
+dist: bionic
 env:
   global:
     - PYTHON=python
 before_install:
-  - sudo apt-add-repository -y "deb http://packages.ros.org/ros/ubuntu xenial main"
-  - wget https://raw.githubusercontent.com/ros/rosdistro/master/ros.key -O - | sudo apt-key add -
+  - sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
+  - sudo apt-add-repository -y "deb http://packages.ros.org/ros/ubuntu bionic main"
   - sudo apt-get update
-  - sudo apt-get -y install ros-kinetic-ros-base ros-kinetic-common-msgs
+  - sudo apt-get -y install python-rosdep ros-melodic-ros-base ros-melodic-common-msgs
   - sudo rosdep init
   - rosdep update
 before_script:
   - export PATH=/usr/bin:$PATH
-  - source /opt/ros/kinetic/setup.sh
+  - source /opt/ros/melodic/setup.sh
   - roscore &
   - sleep 5
   - python test/echonode.py &