Fix sandbox command

Rails does not create a sprockets manifest anymore,
but we still depend on it. Since rails commands do not
work anymore, because sprockets raises an error if no
`manifest.js` file is present we need to create it in
order to be able to run `rails new`.

Author: tvdeyen

bin/sandbox

@@ -33,6 +33,14 @@ function unbundled {
 }
 
 rm -rf ./sandbox
+
+mkdir -p sandbox/app/assets/config
+cat <<MANIFEST > sandbox/app/assets/config/manifest.js
+//= link_tree ../images
+//= link_directory ../javascripts .js
+//= link_directory ../stylesheets .css
+MANIFEST
+
 rails new sandbox \
   --database="$RAILSDB" \
   --skip-git \