Assets helper Sprockets 4 compatibility fix.

[martinsabo]

Pathname attr reader was removed/deprecated. Only filename is available in Sprockets 4.x.

[rwrrll]

I need this patch, but the Travis build appears to have failed because it couldn't find ARel 8.0. I've just checked & that gem is definitely live on Rubygems now, so is anyone able to retry this build?

[unixmonkey]

I just reran the Travis build and it failed exactly the same way.

Bundler could not find compatible versions for gem "arel":
  In Gemfile:
    rails was resolved to 5.1.0.alpha, which depends on
      activerecord (= 5.1.0.alpha) was resolved to 5.1.0.alpha, which depends on
        arel (~> 8.0)
Could not find gem 'arel (~> 8.0)', which is required by gem 'activerecord (=
5.1.0.alpha)', in any of the sources.
The command "eval bundle install --verbose " failed.

[unixmonkey]

Anyway, I like this change. It's going in. Thanks!