= all: add parens to `sender()`

[The migration guide](http://doc.akka.io/docs/akka/2.3.0-RC1/project/migration-guide-2.2.x-2.3.x.html#Parens_Added_to_sender) says:

> Parens were added to the sender() method of the Actor Scala API to
> highlight that the sender() reference is not referentially transparent
> and must not be exposed to other threads, for example by closing over
> it when using future callbacks.

Although it's not mandatory to use parens, following the new convention
must be a good practice.