Update pubspec.yaml

Update version no as https://github.com/flutter/flutter_markdown/pull/56 was a breaking change (Dart 2 only)
diff --git a/packages/flutter_markdown/pubspec.yaml b/packages/flutter_markdown/pubspec.yaml
index c679a07..d4e6565 100644
--- a/packages/flutter_markdown/pubspec.yaml
+++ b/packages/flutter_markdown/pubspec.yaml
@@ -2,7 +2,7 @@
 author: Flutter Authors <flutter-dev@googlegroups.com>
 description: A Markdown renderer for Flutter.
 homepage: https://github.com/flutter/flutter_markdown
-version: 0.1.7
+version: 0.2.0
 
 dependencies:
   flutter: