Update AHC dependency to 1.8.1

Inc version to 0.6.0-SNAPSHOT, as this is a breaking change, for a
couple of reasons:
 1) removed :raw-url, since .getRawUrl can now throw an exception.
 2) (string resp) now returns an empty string instead of nil. I'm really
    not sure why this behavior changed, but it seems reasonable to me
    for it to always returns a string.

The good news is that version 1.8.1 fixes the websocket bug that caused
both :text and :bytes handlers to be called for each received frame.