Update Masterminds/semver to latest 2.x (deal w/non-pointer Version)

[sdboyer]

We've been lagging behind on our semver dependency for a while now. The main reason is, back in January, i made a PR that 'depointerized' semver.Version objects - Masterminds/semver#27 - but never quite finished adapting gps to rely on the new value-based patterns. Doing this is now a prerequisite for #225, which is part of our manifest/lock stabilization milestone, so we gotta get it done.

I did start in on the conversion, and I remember running into some slightly annoying things, but nothing terrible. It does involve at least looking through some of the more complicated parts of gps' internals, but I don't think it will be horrible to do - I suspect that someone looking to help out could basically do the following:

1. Update dep's semver dependency to the latest 2.x (dep ensure -update)
2. Fix the resulting compile errors
3. Search for any other places dep's code directly references semver.Version and make sure they seem sane

And get [almost] the whole way there. If something weird does come up, we can discuss as it goes along.

[xmattstrongx]

Ill take this on 👍

[sdboyer]

Fixed by #579.