Fix string interpolator error in HeartbeatReceiver

Error log before fixed
<code>15/03/29 10:07:25 ERROR YarnScheduler: Lost an executor 24 (already removed): Executor heartbeat timed out after ${now - lastSeenMs} ms</code>

Author: Li Zhihui <zhihui.li@intel.com>

Closes #5255 from li-zhihui/fixstringinterpolator and squashes the following commits:

c93f2b7 [Li Zhihui] Fix string interpolator error in HeartbeatReceiver

core/src/main/scala/org/apache/spark/HeartbeatReceiver.scala

@@ -89,7 +89,7 @@ private[spark] class HeartbeatReceiver(sc: SparkContext, scheduler: TaskSchedule
         logWarning(s"Removing executor $executorId with no recent heartbeats: " +
           s"${now - lastSeenMs} ms exceeds timeout $executorTimeoutMs ms")
         scheduler.executorLost(executorId, SlaveLost("Executor heartbeat " +
-          "timed out after ${now - lastSeenMs} ms"))
+          s"timed out after ${now - lastSeenMs} ms"))
         if (sc.supportDynamicAllocation) {
           sc.killExecutor(executorId)
         }