Fix SigMF metadata version number formatting, per updated spec

It turns out that the correct version number formatting for SigMF is "X.Y.Z" instead of "vX.Y.Z" (see sigmf/sigmf#298)

We currently use the latter in the `sigmf_builder`

This would presumably be solved by a fix to #29 following sigmf/sigmf-python#21, but a simple change could be made in the meantime