Snowhouse v4.0.1

This patch release gets rid of a -Wnon-virtual-dtor warning when compiling with C++11 support.

Note that this release (and further major version 4 releases) will not provide a headers-only branch, since further maintenance of major version 4 is one a side branch (maint-v4) and version 5.0.0 will be releases in the very near future.