Update URLs after moving to the Flutter organization

packages/flutter/pubspec.yaml

 name: sky
 version: 0.0.46
 author: Chromium Authors <sky-dev@googlegroups.com>
-description: A framework for writing Sky applications
-homepage: https://github.com/domokit/sky_engine/tree/master/sky/packages/sky
+description: A framework for writing Flutter applications
+homepage: http://flutter.io
 dependencies:
   cassowary: ^0.1.7
   material_design_icons: ^0.0.3

packages/flutter_sprites/README.md

 # skysprites
 
-A sprite toolkit built on top of Sky.
+A sprite toolkit built on top of Flutter.
 
 ## Getting Started
 
-For help getting started with Sky, view our online
-[documentation](https://github.com/domokit/sky_engine/blob/master/sky/packages/sky/README.md).
+For help getting started with Flutter, view our online
+[documentation](http://flutter.io).

packages/flutter_sprites/pubspec.yaml

 name: skysprites
-description: A sprite toolkit built on top of Sky.
+description: A sprite toolkit built on top of Flutter
 version: 0.0.1
 author: Chromium Authors <sky-dev@googlegroups.com>
-homepage: https://github.com/domokit/sky_engine/tree/master/sky/packages/sky
+homepage: http://flutter.io
 dependencies:
   sky: ">=0.0.36 < 0.1.0"
   sky_tools: ">=0.0.10 < 0.1.0"