Bump the version number to 0.5.6.
Bump the version number to 0.5.6.

Changes since 0.5.5:
- Modernize the cmake code for linking libraries.
- Allow dicomfind to report number of files via NumberOfReferences.

dgobbi/vtk-dicom,hendradarwin/vtk-dicom,hendradarwin/vtk-dicom,dgobbi/vtk-dicom,hendradarwin/vtk-dicom,dgobbi/vtk-dicom