Add rubber cache files to TeX.gitignore

Generated by [rubber](https://tex-talk.net/2011/12/building-documents-with-rubber/).  [The docs](https://github.com/petrhosek/rubber/blob/8ec18fd096b186901f197d26c8e1060b42f0b34f/doc/rubber.texi#L158) claim to use `rubber.cache`, but I've seen $filename.rubbercache in the wild, so I'm including both.