0.0.7: support clojure.core.reducers/fold, depend on Clojure 1.5

fold is supported using RRB-Tree slice operation.

The coll-fold implementation for c.l.IPersistentVector provided by the
reducers library uses clojure.core/subvec instead; currently it
wouldn't work here due to assumptions made by
c.l.APersistentVector$SubVector. (Incidentally, the same assumptions
break it for gvec.)