Revert soname bump

This must have been a mistake, since the C++ library hasn't changed at all. Best to only increment age in tandem with current.

libtool versioning is silly and confusing, so I have a comment above those variables explaining one good strategy for maintaining it.

Only accept this pull request if you plan to do a fixup release right away (which I would recommend, to save trouble for distributions); otherwise, downstreams can just rebuild gnome-sudoku.