citation() function in Eidos needs to be updated to give the SLiM 5 paper citation

[bhaller]

We've got a DOI for the SLiM 5 paper at MBE now, so the citation() function should be updated to provide that information. (Should've done this for the bioRxiv preprint already, I suppose!)

[bhaller]

I did this for the SLiM 5 paper just now, but we don't have a volume/issue yet, so I'll leave this issue open and revisit it when the paper comes out.