SPARK-1337: Application web UI garbage collects newest stages

Simple fix...

Author: Patrick Wendell <pwendell@gmail.com>

Closes #320 from pwendell/stage-clean-up and squashes the following commits:

29be62e [Patrick Wendell] SPARK-1337: Application web UI garbage collects newest stages instead old ones
(cherry picked from commit ee6e9e7d863022304ac9ced405b353b63accb6ab)

Conflicts:

	core/src/main/scala/org/apache/spark/ui/jobs/JobProgressListener.scala
	core/src/test/scala/org/apache/spark/ui/jobs/JobProgressListenerSuite.scala