Fixing orocos catkin detection.

Previously, it was required that find_package(catkin) was called before
including orocos's use-file so that it could know you wanted to use
caktkin. Now, instead, if there is a package.xml file in the root
directory of the project, it tries to find catkin, and if catkin is
found, it enables ORO_USE_CATKIN.
trying to fix travis test config
Merge branch 'remove_duplicates_from_use_orocos_variables-updated' into catkin