Change version number to 0.25-preview
Change version number to 0.25-preview

Our new version numbering system is quite clever and tries to advance
the teeny version number automagically.  Instead of versioning
prereleases as `0.25.x` where `x` is monotonically increasing, version
our prereleases as `0.25-preview`.

This is roughly what our previous strategy looked like.

Zoxive/libgit2sharp,Zoxive/libgit2sharp,PKRoma/libgit2sharp,libgit2/libgit2sharp