Roll Observatory packages and add a roll script

packages/barback/example/markdown_converter/pubspec.yaml

+name: markdown_converter
+description: This hello world example implements a simple
+             transformer that converts a markdown file (with
+             a ".mdown", ".md", or a ".markdown" extension) to HTML.
+
+dependencies:
+  barback: ">=0.14.1 <0.16.0"
+  markdown: any
+
+# Override the barback dependency so this example always uses the version
+# of barback it's bundled with.
+dependency_overrides:
+  barback: {path: ../..}
+
+transformers:
+- markdown_converter