Update compute_asset_path to Rails 4 signature
Update compute_asset_path to Rails 4 signature

Fixes https://github.com/searls/jasmine-rails/issues/213

The JasmineRails::OfflineAssetPaths monkey patch of
`compute_asset_path` has a Rails 3 signature:
https://github.com/searls/jasmine-rails/blob/master/lib/jasmine_rails/of
fline_asset_paths.rb#L9

This was updated from 3 arguments to 2 in Rails 4:
https://apidock.com/rails/ActionView/AssetPaths/compute_public_path
vs
https://apidock.com/rails/ActionView/Helpers/AssetUrlHelper/compute_asse
t_path

If JasmineRails::OfflineAssetPaths.disabled is true, then the method is
meant to shortcut and run the original super method. This gives an
error like:

ActionView::Template::Error: wrong number of arguments (given 3,
expected 1..2)

* Update `compute_asset_path` to Rails 4 signature
* Use explicit arguments for `compute_public_path`
* Ignore unused `dir` variable from Rails 3

searls/jasmine-rails,searls/jasmine-rails,searls/jasmine-rails