[SPARK-6998][MLlib] Make StreamingKMeans 'Serializable'

If `StreamingKMeans` is not `Serializable`, we cannot do checkpoint for applications that using `StreamingKMeans`. So we should make it `Serializable`.

Author: zsxwing <zsxwing@gmail.com>

Closes #5582 from zsxwing/SPARK-6998 and squashes the following commits:

67c2a14 [zsxwing] Make StreamingKMeans 'Serializable'

mllib/src/main/scala/org/apache/spark/mllib/clustering/StreamingKMeans.scala

@@ -165,7 +165,7 @@ class StreamingKMeansModel(
 class StreamingKMeans(
     var k: Int,
     var decayFactor: Double,
-    var timeUnit: String) extends Logging {
+    var timeUnit: String) extends Logging with Serializable {
 
   def this() = this(2, 1.0, StreamingKMeans.BATCHES)