pinhole_camera_model: fix implicit shared_ptr cast to bool for C++11

In C++11 boost::shared_ptr does not provide the implicit bool conversion
operator anymore, so make the cast in pinhole_camera_model.h explicit.
That does not hurt in older C++ standards and makes compilation with C++11
possible.
Merge pull request #37 from xqms/boolconv_fix

pinhole_camera_model: fix implicit shared_ptr cast to bool for C++11