Depend on boost

Boost is clearly needed by this package [1], so it should depend on it. This probably came to light because an upstream package no longer depends on boost, or the dependencies of this package changed. In any case, it should be added.

Example of build failure: https://csc.mcs.sdsmt.edu/jenkins/job/ros-indigo-class-loader_binaryrpm_21_x86_64/2/console

Associated rosdep key: https://github.com/ros/rosdistro/blob/master/rosdep/base.yaml#L277-L324

Thanks,

--scott

[1] https://github.com/ros/class_loader/blob/indigo-devel/CMakeLists.txt#L4