Tag style changed for 0.5.1 release

[DrDaveD]

The tag v0.5.1 was accidentally used for the 0.5.1 release, where previous releases used tags without the v. That can break some build scripts, as was mentioned in #129 (although apparently deleted; I got the email but the comment isn't there anymore).

[DrDaveD]

I created an additional 0.5.0 tag but don't know if it's safe to change the existing release to use it or if I should make an 0.5.2 just to fix the tag.