SPARK-1365 [HOTFIX] Fix RateLimitedOutputStream test

This test needs to be fixed. It currently depends on Thread.sleep() having exact-timing
semantics, which is not a valid assumption.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #277 from pwendell/rate-limited-stream and squashes the following commits:

6c0ff81 [Patrick Wendell] SPARK-1365: Fix RateLimitedOutputStream test