Require build_config ^0.2.6, simplify config (#132)

Author: kevmoo

json_serializable/build.yaml

@@ -1,7 +1,6 @@
 # Read about `build.yaml` at https://pub.dartlang.org/packages/build_config
 builders:
   json_serializable:
-    target: ":json_serializable"
     import: "package:json_serializable/builder.dart"
     builder_factories: ["jsonSerializable"]
     build_extensions: {".dart": [".g.dart"]}

json_serializable/pubspec.yaml

@@ -8,7 +8,7 @@ environment:
 dependencies:
   analyzer: '>=0.29.10 <0.32.0'
   build: '>=0.9.0 <0.13.0'
-  build_config: ^0.2.1
+  build_config: ^0.2.6
   cli_util: ^0.1.0
 
   # Use a tight version constraint to ensure that a constraint on