Replaced the deprecated RaisedButton with the equivalent ElevatedButton. (#127)

Author: zinigor

README.md

@@ -68,9 +68,9 @@ void main() {
       });
 
   dashbook
-      .storiesOf('RaisedButton')
+      .storiesOf('ElevatedButton')
       .decorator(CenterDecorator())
-      .add('default', (ctx) => RaisedButton(child: Text('Ok'), onPressed: () {}));
+      .add('default', (ctx) => ElevatedButton(child: Text('Ok'), onPressed: () {}));
 
   // Since dashbook is a widget itself, you can just call runApp passing it as a parameter
   runApp(dashbook);