Add flutter 1.20.0 requirement to pubspec (#423)

MrAlek · web-flow · commit e133e680e4c7 · 2020-08-18T23:58:08.000+03:00
diff --git a/pubspec.yaml b/pubspec.yaml
@@ -5,6 +5,7 @@ homepage: https://github.com/danvick/flutter_form_builder
 
 environment:
   sdk: ">=2.6.0 <3.0.0"
+  flutter: ^1.20.0 
 
 dependencies:
   flutter:
