Eliminate the need for an on_load block

Addreses https://github.com/rails/rails/issues/9461

Given that sprockets-rails/lib/sprockets/railtie.rb contains:

  require 'sprockets/rails/helper'

And sprockets-rails/lib/sprockets/ruby/helper.rb starts out with:

  require 'action_view'

It follows that an on_load hander is not required.
Merge pull request #43 from rubys/rails_issues_9461

Eliminate the need for an on_load block
Remove journey since it was merged into Rails, and change deprecated rubygems source
Revert "Merge pull request #43 from rubys/rails_issues_9461"

This reverts commit ccf92ee40cb4c9a966a22106a721f7364b5fc6c9, reversing
changes made to 8105eb9ee2d4889f9d3d7c327899b63493f4b061.

Reason: Helper module should be included into AV::Base, extending it with
the helpers and config options.
Replace on_load with instance_eval

Addresses https://github.com/rails/rails/issues/9461
Merge pull request #44 from rubys/rails_issue_9461

Replace on_load with instance_eval
Bump sprockets-rails to rc4
Merge branch 'master' of https://github.com/rovermicrover/sprockets-rails