Build warnings

[peller]

No description provided.

[peller]

@poplav: Some other (non-fatal) warnings and errors to look at, mostly from Scala:

16/09/22 16:52:23 [ERROR] o.a.s.u.Utils - Uncaught exception in thread driver-heartbeater java.io.IOException: java.lang.ClassCastException: cannot assign instance of scala.collection.immutable.HashMap$SerializationProxy to field org.apache.spark.executor.TaskMetrics._accumulatorUpdates of type scala.collection.immutable.Map in instance of org.apache.spark.executor.TaskMetrics

also

com.fasterxml.jackson.core.JsonParseException: Unrecognized token 'not': was expecting 'null', 'true', 'false' or NaN (includes a stack trace)

That section includes lots of [WARN] and [ERROR] conditions which may be part of the test. Perhaps they should be redirected away from the logs if the output is considered normal?

[poplav]

So far I found: In kernel-scala/src/main/scala/declarativewidgets/util/Explore.scala changing:
val valDefTrees = requestTrees collect {case c: ValDef => c} to
val valDefTrees = requestTrees.map(_.asInstanceOf[ValDef]) gets rid of the one warning left.

[peller]

Refs #517

[peller]

@lbustelo ready for review. Switch to FF for now, perhaps leave #517 open until we identify an issue for chrome/linux