Rack::StaticCache: use semver for asset versioning

Unfortunately the previous regex matched files ending with - followed by any number. This comes up a lot and I'm getting 404s for dozens of our files.

I suggest using a simplified semvar of `x.x.x` (x being 0-9) (unless we want to use the [full semver spec regex](https://github.com/mojombo/semver/issues/32#issuecomment-7663411)).

This passes the current versioning tests for a `-1.0.0` ending. Feel free to try the regex out at [this rubular test](http://rubular.com/r/sZ2raKn98v) (using $ in rubular example to allow for multiple matches).