fix the apply method

@williamboxhall The ```()``` was missing for ```apply```. The consequence is that when using ```clump.apply()```, you are actually using the Clump's ```apply``` and then the Future's ```apply```[1], that is a blocking operation.

[1] https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Future.scala#L655