Add preference version, auto-update old prefs
This will prevent users from having insanely low thresholds when they
update to the version that does not store acceleration in terms of g.

This also includes a version update because otherwise it would be awkward.

This commit is untested right now; I won't push it until after I've tested
it, but I can't really test it without committing because I need to
install an old version first.
Merge branch 'master' of https://github.com/SyntaxBlitz/PluckLock