Return emissary subscription directly

src/config-observer.coffee

@@ -5,5 +5,5 @@ module.exports =
 
   unobserveConfig: ->
     if @configSubscriptions?
-      subscription.cancel() for keyPath, subscription of @configSubscriptions
+      subscription.off() for keyPath, subscription of @configSubscriptions
       @configSubscriptions = null

src/config.coffee

@@ -216,8 +216,7 @@ class Config
         callback(value, {previous})
 
     eventName = "updated.#{keyPath.replace(/\./, '-')}"
-    subscription = { cancel: => @off eventName, updateCallback  }
-    @on eventName, updateCallback
+    subscription = @on eventName, updateCallback
     callback(value) if options.callNow ? true
     subscription