Add Travis CI config. Check Indigo, Kinetic, Melodic.

[130s]

No description provided.

[130s]

Travis CI seems not enabled for this repo. Could any admin enable (it's just one click) at https://travis-ci.org/ros-perception/depthimage_to_laserscan?

[130s]

To see the result of the change in this PR, I enabled Travis CI on my fork https://travis-ci.org/130s/depthimage_to_laserscan/builds/459582606.

Only Melodic jobs fail (which I think #27 addresses) https://travis-ci.org/130s/depthimage_to_laserscan/jobs/459582611#L579

Errors     << depthimage_to_laserscan:make /root/catkin_ws/logs/depthimage_to_laserscan/build.make.000.log
/root/catkin_ws/src/depthimage_to_laserscan/src/DepthImageToLaserScanNodelet.cpp:60:24: error: expected constructor, destructor, or type conversion before ‘(’ token
 PLUGINLIB_DECLARE_CLASS(depthimage_to_laserscan, DepthImageToLaserScanNodelet, depthimage_to_laserscan::DepthImageToLaserScanNodelet, nodelet::Nodelet);
                        ^
make[2]: *** [CMakeFiles/DepthImageToLaserScanNodelet.dir/src/DepthImageToLaserScanNodelet.cpp.o] Error 1
make[1]: *** [CMakeFiles/DepthImageToLaserScanNodelet.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [all] Error 2

[clalancette]

Given that Travis no longer does open-source testing, I'm going to close this one out.