[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.

core/src/main/scala/org/apache/spark/ui/JettyUtils.scala

@@ -29,8 +29,8 @@ import org.eclipse.jetty.client.api.Response
 import org.eclipse.jetty.proxy.ProxyServlet
 import org.eclipse.jetty.server._
 import org.eclipse.jetty.server.handler._
+import org.eclipse.jetty.server.handler.gzip.GzipHandler
 import org.eclipse.jetty.servlet._
-import org.eclipse.jetty.servlets.gzip.GzipHandler
 import org.eclipse.jetty.util.component.LifeCycle
 import org.eclipse.jetty.util.thread.{QueuedThreadPool, ScheduledExecutorScheduler}
 import org.json4s.JValue

pom.xml

@@ -136,7 +136,7 @@
     <derby.version>10.12.1.1</derby.version>
     <parquet.version>1.8.2</parquet.version>
     <hive.parquet.version>1.6.0</hive.parquet.version>
-    <jetty.version>9.2.16.v20160414</jetty.version>
+    <jetty.version>9.3.11.v20160721</jetty.version>
     <javaxservlet.version>3.1.0</javaxservlet.version>
     <chill.version>0.8.0</chill.version>
     <ivy.version>2.4.0</ivy.version>