Fix error thrown when updating Observable listeners

Non-MWE:
- In IJulia, with PlotlyJS, an observer on a plot event (e.g.
  "hover", "click", etc) gave the following error when updating/being run:

```julia
KERNEL EXCEPTION
MethodError: objects of type Pair{Int64, Any} are not callable

Stacktrace:
  [1] #invokelatest#2
    @ ./essentials.jl:729 [inlined]
  [2] invokelatest
    @ ./essentials.jl:726 [inlined]
  [3] set_nosync(ob::Observable{Dict{Any, Any}}, val::Dict{String, Any})
    @ WebIO ~/.julia/dev/WebIO/src/scope.jl:355
  [4] dispatch(ctx::Scope, key::String, data::Dict{String, Any})
    @ WebIO ~/.julia/dev/WebIO/src/scope.jl:368
  [5] dispatch_command(conn::WebIO.IJuliaConnection, data::Dict{String, Any})
    @ WebIO ~/.julia/dev/WebIO/src/messaging.jl:104
  [6] dispatch(conn::WebIO.IJuliaConnection, data::Dict{String, Any})
    @ WebIO ~/.julia/dev/WebIO/src/messaging.jl:81
  [7] (::WebIO.var"#97#98"{WebIO.IJuliaConnection})(msg::IJulia.Msg)
    @ WebIO ~/.julia/dev/WebIO/src/providers/ijulia.jl:21
  [8] comm_msg(sock::ZMQ.Socket, msg::IJulia.Msg)
    @ IJulia.CommManager ~/.julia/packages/IJulia/AQu2H/src/comm_manager.jl:134
  [9] #invokelatest#2
    @ ./essentials.jl:729 [inlined]
 [10] invokelatest
    @ ./essentials.jl:726 [inlined]
 [11] eventloop(socket::ZMQ.Socket)
    @ IJulia ~/.julia/packages/IJulia/AQu2H/src/eventloop.jl:8
 [12] (::IJulia.var"#15#18")()
    @ IJulia ./task.jl:484
```

I narrowed the problem down to an incorrect assumption about what is
returned from `Observables.listeners`. The previous expectation was an
array of functions, the reality is a `Vector{Pair,Function}`.

Author: halleysfifthinc

Project.toml

@@ -24,7 +24,7 @@ AssetRegistry = "0.1.0"
 FunctionalCollections = "0.5.0"
 JSExpr = "0.5"
 JSON = "0.18, 0.19, 0.20, 0.21"
-Observables = "0.3, 0.4, 0.5"
+Observables = "0.4, 0.5"
 Requires = "0.4.4, 0.5, 1.0.0"
 WebSockets = "1.5.0"
 Widgets = "0.6.2"

src/scope.jl

@@ -332,31 +332,21 @@ struct SyncCallback
 end
 
 (s::SyncCallback)(xs...) = s.f(xs...)
+
 """
 Set observable without synchronizing with the counterpart on the browser.
 
 This is mostly used to update observables in response to updates sent from th
 browser (so that we aren't sending the same update *back* to the browser).
 """
-function set_nosync end
-
-if isdefined(Observables, :setexcludinghandlers)
-    # Observables <=0.3
-    function set_nosync(ob, val)
-        Observables.setexcludinghandlers(ob, val, x -> !(x isa SyncCallback))
-        return
-    end
-else
-    # Observables >=0.4
-    function set_nosync(ob, val)
-        Observables.setexcludinghandlers!(ob, val)
-        for f in listeners(ob)
-            if !(f isa SyncCallback)
-                Base.invokelatest(f, val)
-            end
+function set_nosync(ob, val)
+    Observables.setexcludinghandlers!(ob, val)
+    for (_, f) in listeners(ob)
+        if !(f isa SyncCallback)
+            Base.invokelatest(f, val)
         end
-        return
     end
+    return
 end
 
 const lifecycle_commands = ["scope_created"]