Use new Configuration() instead of new JobConf() for ObjectWritable.

JobConf's constructor loads default config files in some verisons of
Hadoop, which is quite slow, and we only need the Configuration object
to pass the correct ClassLoader.

core/src/main/scala/spark/SerializableWritable.scala

@@ -21,7 +21,7 @@ import java.io._
 
 import org.apache.hadoop.io.ObjectWritable
 import org.apache.hadoop.io.Writable
-import org.apache.hadoop.mapred.JobConf
+import org.apache.hadoop.conf.Configuration
 
 class SerializableWritable[T <: Writable](@transient var t: T) extends Serializable {
   def value = t
@@ -35,7 +35,7 @@ class SerializableWritable[T <: Writable](@transient var t: T) extends Serializa
   private def readObject(in: ObjectInputStream) {
     in.defaultReadObject()
     val ow = new ObjectWritable()
-    ow.setConf(new JobConf())
+    ow.setConf(new Configuration())
     ow.readFields(in)
     t = ow.get().asInstanceOf[T]
   }