[SPARK-14897][CORE] Upgrade Jetty to latest version of 8

## What changes were proposed in this pull request?

Update Jetty 8.1 to the latest 2016/02 release, from a 2013/10 release, for security and bug fixes. This does not resolve the JIRA necessarily, as it's still worth considering an update to 9.3.

## How was this patch tested?

Jenkins tests

Author: Sean Owen <sowen@cloudera.com>

Closes #12842 from srowen/SPARK-14897.