Allow return statement when parsing bindings

An explicit return statement was allowed in bindings with v8 (as
they were rewritten as function closures. There's no reason to forbid them
now.

Fixes two tests in qqmlecmascript.

Change-Id: I27a6f88d17d8c35b37735ad321b7607f09b1e67c
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>