Fix for `put!` docstring: closed gives false

Signed-off-by: puredanger <alex.miller@cognitect.com>
Mention how to get access to snapshot releases.

per ASYNC-88
Removed last change since we don't actually do snapshot releases
comment out invalid protocol implementation, was picked up by improved
var analysis in ClojureScript
move cljs.core.async.impl.ioc-helpers/ioc-alts! to
cljs.core.asycn/ioc-alts! same as clojure.core.async. flatmap ->
mapcat. cljs dep bump.
Update latest version in README
Support (try (catch :default)) in CLJS exception handling
bump CLJS
merge