Adjust typespecs for Erlang R17+

[pnc]

Hi! I totally get if you don't wanna think about this project anymore, but I went to build jerg on Erlang R18 and the compilation bombed out with:

rebar get-deps compile
==> jsx (get-deps)
==> kvc (get-deps)
==> getopt (get-deps)
==> jerg (get-deps)
==> jsx (compile)
==> kvc (compile)
/Users/me/gammaray/jerg/deps/kvc/src/kvc.erl:15: type gb_tree() undefined
Compiling /Users/me/gammaray/jerg/deps/kvc/src/kvc.erl failed:
ERROR: compile failed while processing /Users/me/gammaray/jerg/deps/kvc: rebar_abort

This was an easy fix: upgrade kvc, then update a few typespecs in jerg.erl.