Support PHP 7.1 `void` return type

[Ocramius]

As per PHP 7.1, a new void return type is introduced ( https://wiki.php.net/rfc/void_return_type )

This basically means that the TypeGenerator needs to be patched to support void.

No further changes really needed in this component, as it doesn't really focus on method body generation.

[michalbundyra]

Done in #87 and released with 3.1.0.