Updated PostgisBuilder \Closure instead of Closure

fixes composer error:

[ErrorException]                                                                                                                                                                                                                            
  Argument 2 passed to Lamb\LaravelPostgisDatabase\PostgisBuilder::createBlueprint() must be an instance of Lamb\LaravelPostgisDatabase\Closure, instance of Closure given, called in .../vendor/laravel/framework  
  /src/Illuminate/Database/Schema/Builder.php on line 94 and defined