gripper_action_controller: drop unneeded rostest dependency
diff_drive_controller: add realtime_tools dependency

When executing 'catkin_make -DCATKIN_ENABLE_TESTING=0', this error occurs:

In file included from [...]/diff_drive_controller/src/diff_drive_controller.cpp:45:0:
[...]/diff_drive_controller/include/diff_drive_controller/diff_drive_controller.h:46:44: fatal error: realtime_tools/realtime_buffer.h: No such file or directory
 #include <realtime_tools/realtime_buffer.h>
                                            ^
compilation terminated.
make[2]: *** [ros_controllers/diff_drive_controller/CMakeFiles/diff_drive_controller.dir/src/diff_drive_controller.cpp.o] Error 1

Obviously, the realtime_tools dependency was missing in the CMakeLists.txt file.
joint_trajectory_controller: make rostest in CMakeLists optional (ros/rosdistro#3010)
Merge pull request #113 from bulwahn/indigo-devel

addressing test dependencies with -DCATKIN_ENABLE_TESTING=0