Add iterable return type to getReadRoutes() and getWriteRoutes() #19
Description
In PHP 7.1 the iterable pseudo type will be introduced that embraces the array and \Traversable types.
Currently both mentioned methods do not have a return type as we wanted the user to decide, whether an array or an \Iterator is returned.
When targeting PHP 7.1 with the next major version 3.0.0 the iterable return type can be added.
This also affects the ConfigWrapper class and lets the guardRoutesAreTraversable() method in AbstractRouter become obsolete.