https://github.com/jsk-ros-pkg/jsk_common/pull/387 was wrong.

we should not SKIP_PKG_CONFIG,
https://github.com/jsk-ros-pkg/jsk_common/pull/387#issuecomment-40590011 was wrong
we need to generate assimp_devel.pc using catkin_package, because when catkin_make runs, it uses 
`pkg_check_modules(assimp_devel assimp_devel REQUIRED)`, this means we need assimp_devel.pc before make runs and `catkin_package()` do this job.