Update CHANGELOG with recent changes to Sky's pub package

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1104093004

packages/flutter/CHANGELOG.md

+## 0.0.5+dart-summit-5
+
+  - Added sky_tool start_tracing and stop_tracing.
+
+## 0.0.5+dart-summit-4
+
+  - Added download_material_design_icons script.
+
 ## 0.0.5+dart-summit-2
 
   - Various demo fixes and added ChangeLogs.
 
-
 ## 0.0.5+dart-summit-1
 
   - Branched from mojo trunk to stabalize for Dart summit.
 
-
 ## 0.0.2
 
   - sdk_additions now includes previously missing imports.