[graphX] GraphOps: random pick vertex bug

When `numVertices > 50`, probability is set to 0. This would cause infinite loop.

Author: yingjieMiao <yingjie@42go.com>

Closes #2553 from yingjieMiao/graphx and squashes the following commits:

6adf3c8 [yingjieMiao] [graphX] GraphOps: random pick vertex bug

(cherry picked from commit 51229ff7)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>

graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala

@@ -254,7 +254,7 @@ class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) extends Seriali
    * Picks a random vertex from the graph and returns its ID.
    */
   def pickRandomVertex(): VertexId = {
-    val probability = 50 / graph.numVertices
+    val probability = 50.0 / graph.numVertices
     var found = false
     var retVal: VertexId = null.asInstanceOf[VertexId]
     while (!found) {