Drop ROS Indigo from CI

[sloretz]

CI on #65 failed for trusty/indigo. Since ROS Indigo is EOL, I'm hoping it is ok to drop CI for it instead of spending time investigating the issue.

++ cd ..
++ rosdep install -q -n --from-paths src --ignore-src --rosdistro indigo -y
ERROR: the following packages/stacks could not have their rosdep keys resolved
to system dependencies:
rosgraph_msgs: Cannot locate rosdep definition for [message_runtime]
nav_msgs: Cannot locate rosdep definition for [message_runtime]
shape_msgs: Cannot locate rosdep definition for [message_runtime]
std_srvs: Cannot locate rosdep definition for [message_runtime]
stereo_msgs: Cannot locate rosdep definition for [message_runtime]
std_msgs: Cannot locate rosdep definition for [message_runtime]
actionlib_msgs: Cannot locate rosdep definition for [message_runtime]
trajectory_msgs: Cannot locate rosdep definition for [rosbag_migration_rule]
sensor_msgs: Cannot locate rosdep definition for [message_runtime]
geneus: Cannot locate rosdep definition for [genmsg]
diagnostic_msgs: Cannot locate rosdep definition for [message_runtime]
geometry_msgs: Cannot locate rosdep definition for [message_runtime]
visualization_msgs: Cannot locate rosdep definition for [message_runtime]
actionlib: Cannot locate rosdep definition for [rosnode]
+++ error
+++ '[' /home/travis/ros/ws_geneus/build/test_results ']'
+++ find /home/travis/ros/ws_geneus/build/test_results -type f -exec echo '=== {} ===' ';' -exec cat '{}' ';'
find: `/home/travis/ros/ws_geneus/build/test_results': No such file or directory
+++ rostest -t roseus test-genmsg.catkin.test
.travis.sh: line 8: rostest: command not found
+++ rostest -t roseus test-genmsg-oneworkspace.catkin.launch
.travis.sh: line 9: rostest: command not found
+++ exit 1
The command "docker run -v $HOME:$HOME -e CI_SOURCE_PATH -e REPOSITORY_NAME -e HOME -e DISTRO -e ROS_DISTRO -e BUILDER -e ROS_PARALLEL_JOBS $DOCKER_IMAGE bash -c 'cd $CI_SOURCE_PATH; source .travis.sh'" exited with 1.```

[k-okada]

Closed via #69