Switch to Finn fork of sprockets-rails

See https://github.com/finnlabs/sprockets-rails/commit/bc666f14361d4a8e50af813835527c43d58e539f

Bug # 12284

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Serve angular-busy.html through asset pipeline

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #1523 from opf/fix/assets-relative-url-root

Fix assets with Rails relative url root