Update README.md to include Rails 4.0+ installation

As of Rails 4, the :assets group is no longer supported in the Gemfile ([source](http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0)).  When the :assets group is included, the Gem is simply not included to the project (therefore doesn't work).

Thank you!