Adding an aggregate transformer example to barback. Also updating the previous examples.

pkg/barback/example/simple_transformer/pubspec.yaml

 name: simple_transformer
 description: This hello world example implements a simple
              transformer that inserts a copyright string into
              an input asset - a file that ends with ".txt".
+
 dependencies:
-  barback: any
+  barback: ">=0.14.1 <0.16.0"
+
+# Override the barback dependency so this example always uses the version
+# of barback it's bundled with.
+dependency_overrides:
+  barback: {path: ../..}
 
 transformers:
 - simple_transformer