Fixed too frequent job cleanups

The ImportingManager cleans up jobs that has not been touched in 60ms.
According to comment this should be 60 minutes but was changed in
4529310237.

main/src/com/google/refine/importing/ImportingManager.java

@@ -110,7 +110,7 @@ public class ImportingManager {
     static private ScheduledExecutorService service;
 
     final static private long TIMER_PERIOD = 10; // 10 minutes
-    final static private long STALE_PERIOD = 60; // 60 minutes
+    final static private long STALE_PERIOD = 60 * 60 * 1000; // 60 minutes in milliseconds
     
     static private class CleaningTimerTask implements Runnable {
         @Override