Require kinetic checks to pass and add optional melodic checks to Travis CI (#406)

.travis.yml

-sudo: required 
-dist: trusty 
-language: generic 
+sudo: required
+dist: trusty
+language: generic
 compiler:
   - gcc
 notifications:
@@ -17,13 +17,16 @@ env:
     - USE_DEB=true  ROS_DISTRO="kinetic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros/ubuntu
     - USE_DEB=true  ROS_DISTRO="kinetic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros-shadow-fixed/ubuntu
     - USE_DEB=true  ROS_DISTRO="kinetic"   PRERELEASE=true
+    - USE_DEB=true  ROS_DISTRO="melodic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros/ubuntu
+    - USE_DEB=true  ROS_DISTRO="melodic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros-shadow-fixed/ubuntu
+    - USE_DEB=true  ROS_DISTRO="melodic"   PRERELEASE=true
 matrix:
   allow_failures:
-    - env: USE_DEB=true  ROS_DISTRO="kinetic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros/ubuntu
-    - env: USE_DEB=true  ROS_DISTRO="kinetic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros-shadow-fixed/ubuntu
-    - env: USE_DEB=true  ROS_DISTRO="kinetic"   PRERELEASE=true
+    - env: USE_DEB=true  ROS_DISTRO="melodic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros/ubuntu
+    - env: USE_DEB=true  ROS_DISTRO="melodic"   ROS_REPOSITORY_PATH=http://packages.ros.org/ros-shadow-fixed/ubuntu
+    - env: USE_DEB=true  ROS_DISTRO="melodic"   PRERELEASE=true
 install:
   - git clone https://github.com/ros-industrial/industrial_ci.git .ci_config
-script: 
+script:
   - source .ci_config/travis.sh
-#  - source ./travis.sh  # Enable this when you have a package-local script 
+#  - source ./travis.sh  # Enable this when you have a package-local script