Removed incorrect StreamOutput type hint

StreamOutput stub contained an incorrect type hint. I have
removed it so it's compatible with the actual php_riak library.

Before: public function receive(Output $response);
Now:    public function receive($response);

The typehint in PHPDoc comment is still there so the IDE
keeps hinting the Output methods for $response variable.
Merge pull request #2 from pavelsmolka/master

Removed incorrect StreamOutput type hint