[SPARK-5343][GraphX]: ShortestPaths traverses backwards

Corrected the logic with ShortestPaths so that the calculation will run forward rather than backwards. Output before looked like:

```scala
import org.apache.spark.graphx._
val g = Graph(sc.makeRDD(Array((1L,""), (2L,""), (3L,""))), sc.makeRDD(Array(Edge(1L,2L,""), Edge(2L,3L,""))))
lib.ShortestPaths.run(g,Array(3)).vertices.collect
// res0: Array[(org.apache.spark.graphx.VertexId, org.apache.spark.graphx.lib.ShortestPaths.SPMap)] = Array((1,Map()), (3,Map(3 -> 0)), (2,Map()))
lib.ShortestPaths.run(g,Array(1)).vertices.collect
// res1: Array[(org.apache.spark.graphx.VertexId, org.apache.spark.graphx.lib.ShortestPaths.SPMap)] = Array((1,Map(1 -> 0)), (3,Map(1 -> 2)), (2,Map(1 -> 1)))
```

And new output after the changes looks like:

```scala
import org.apache.spark.graphx._
val g = Graph(sc.makeRDD(Array((1L,""), (2L,""), (3L,""))), sc.makeRDD(Array(Edge(1L,2L,""), Edge(2L,3L,""))))
lib.ShortestPaths.run(g,Array(3)).vertices.collect
// res0: Array[(org.apache.spark.graphx.VertexId, org.apache.spark.graphx.lib.ShortestPaths.SPMap)] = Array((1,Map(3 -> 2)), (2,Map(3 -> 1)), (3,Map(3 -> 0)))
lib.ShortestPaths.run(g,Array(1)).vertices.collect
// res1: Array[(org.apache.spark.graphx.VertexId, org.apache.spark.graphx.lib.ShortestPaths.SPMap)] = Array((1,Map(1 -> 0)), (2,Map()), (3,Map()))
```

Author: Brennon York <brennon.york@capitalone.com>

Closes #4478 from brennonyork/SPARK-5343 and squashes the following commits:

aa57f83 [Brennon York] updated to set ShortestPaths to run 'forward' rather than 'backward'

graphx/src/main/scala/org/apache/spark/graphx/lib/ShortestPaths.scala

@@ -61,8 +61,8 @@ object ShortestPaths {
     }
 
     def sendMessage(edge: EdgeTriplet[SPMap, _]): Iterator[(VertexId, SPMap)] = {
-      val newAttr = incrementMap(edge.srcAttr)
-      if (edge.dstAttr != addMaps(newAttr, edge.dstAttr)) Iterator((edge.dstId, newAttr))
+      val newAttr = incrementMap(edge.dstAttr)
+      if (edge.srcAttr != addMaps(newAttr, edge.srcAttr)) Iterator((edge.srcId, newAttr))
       else Iterator.empty
     }