FileSizeBuckets in correct units (dotnet#387)

* Minor telemetry change to log in correct units and make our life easier in the future

* Use Ceiling instead of Round

src/mlnet/Telemetry/MlTelemetry.cs

@@ -35,7 +35,7 @@ public void LogAutoTrainMlCommand(string dataFileName, string task, long dataFil
 
             var telemetry = new Telemetry();
 
-            var fileSizeBucket = Math.Ceiling(Math.Log(dataFileSize, 2));
+            var fileSizeBucket = Math.Pow(2, Math.Ceiling(Math.Log(dataFileSize, 2)));
 
             var fileNameHash = string.IsNullOrEmpty(dataFileName) ? string.Empty : Sha256Hasher.Hash(dataFileName);