Update jsonexample (#72)

- use latest dependencies
- update build.yaml for removed option
- rebuilt generated code

jsonexample/pubspec.yaml

@@ -2,11 +2,11 @@ name: jsonexample
 description: A demonstration of JSON parsing
 
 environment:
-  sdk: '>=2.0.0-dev <3.0.0'
+  sdk: '>=2.1.0 <3.0.0'
 
 dependencies:
-  json_annotation: ^1.0.0
-  built_collection: '>=2.0.0 <5.0.0'
+  json_annotation: ^2.0.0
+  built_collection: ^4.0.0
   built_value: ^6.1.5
 
   flutter:
@@ -15,7 +15,7 @@ dependencies:
 dev_dependencies:
   build_runner: ^1.0.0
   built_value_generator: ^6.1.5
-  json_serializable: ^1.0.0
+  json_serializable: ^2.2.1
 
   flutter_test:
     sdk: flutter