Wrap: Issue with "Web References" header

[skreutzer]

There's an issue with the vertical placement of the "Web References" sometimes, because it gets separated from the actual reference list and ends up on a page before. Most likely the reason for this is that the references are generated as an index, which is a separate appendix (maybe even part) under memoir, and the current attempt to place the header manually is already more some kind of a hack, not using the regular mechanism for the appendix/index generation. Maybe some spring/glue/flexible \vspace{} will already do.