[#77] BREAKING - removed server-side dummy `?reply-fn`

The previous behaviour was ostensibly to allow soft failure in the event
of calling a non-existant (nil) reply-fn but:

1. This made the intended `(if ?reply-fn <...>)` behaviour difficult.
2. This left `?reply-fn` with a deceptive name (it's never actually nil).
3. The default routers trap (and log) exceptions anyway, so an NPE won't
   be a big deal here.

So this is technically a breaking change IF:
1. You were incorrectly calling `?reply-fn` against non-callback events.
2. You are not using the default exception-trapping router.
Bump deps
v1.2.0