Update ActionView::Digestor#digest for rails 4.1.0

format is not passed as an argument anymore and it only takes a hash
instead of 4 arguments

See https://github.com/rails/rails/commit/767a2b009c07c5c3e4b77a77942f11701f1d8858
Merge pull request #540 from huoxito/rails-4-1-0-digest

Update ActionView::Digestor#digest for rails 4.1.0