[SPARK-20514][CORE] Upgrade Jetty to 9.3.11.v20160721

Upgrade Jetty so it can work with Hadoop 3 (alpha 2 release, in particular).
Without this change, because of incompatibily between Jetty versions,
Spark fails to compile when built against Hadoop 3

## How was this patch tested?
Unit tests being run.

Author: Mark Grover <mark@apache.org>

Closes #17790 from markgrover/spark-20514.