Fix clash with assimp 3.1 in CMake.

This bug is happening on Arch Linux with the latest assimp (3.1). The cause is the same one I described in https://github.com/assimp/assimp/issues/282, i.e. a bunch of code was previously ignored because of a `if(WIN32)` check. Until they fix it and release it, we need to check for Boost after assimp.

Original bug reported on the AUR: https://aur.archlinux.org/packages/ros-hydro-collada-urdf/