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

This help us get a green build on spree api. I'm not sure what could be the caveats for rabl of not using the format any more. Will try to investigate that later on and setup some spec for it. Just want to bring this to you attention guys to see if the change is safe / makes sense.
This probably won't matter for a long time (or ever), but I should point out that this will eventually fail:
```ruby
>> '10.0' >= '4.1' #=> false
```

I think you could do `Gem::Version.new(Rails.version) >= Gem::Version.new('4.1')` instead.