Update manifest.js for sprockets 4.0 compatibility

In sprockets 4.0.0, the `link_directory ../stylesheets .css` directive
causes sprockets to process all `.scss` files in an arbitrary order,
regardless of any `@import` statements. This leads to undefined variable
errors when e.g. the file using a variable is processed before the file
that defines it.

Fix by explicitly rooting the sprockets manifest at `application.css`,
as suggested here:

rails/sprockets#597 (comment)

app/assets/config/manifest.js

@@ -1,2 +1,2 @@
 //= link_tree ../images
-//= link_directory ../stylesheets .css
+//= link application.css