Fix for issue `Fatal error:  Call to undefined method stdClass::getEnd()...

Fix for issue `Fatal error:  Call to undefined method stdClass::getEnd() in Prismic/Fragment/StructuredText.php on line 378`

If there's some reason why there shouldn't be spans in pre formatted blocks, then the spans should probably be discarded with `return new PreformattedBlock($json->text, array(), $label);` ??