Make asset host arity 1.

#### Description

I've been in the midst of trying to figure out why Sass::Rails helpers are returning the wrong asset host. As part of that, I noticed that `AssetHostSelection::AssetProvider` has an arity of 2, though this seemingly breaks callable asset hosts in Rails 3.2+ (ref: rails/rails@d3bc12b27feb96e8799e2bcdc6e52b2704360aa3).

#### Details

**Reviewer(s)**: @ggrossman 

#### Risks

Low. Asset host selection may fail.