Html5hivRails simply provides the html5him.js and html5shim-printshim.js libraries in a rails engine form.
For Rails 3.1 and greater, the files will be added to the asset pipeline and available for you to use. You have the option of html5shiv.js and html5shiv-printshiv.js, depending on if you need to support printing.
Just add this to your Gemfile:
gem 'html5shiv-rails'
and run
bundle install
Since you probably only want to include this when serving IE, place it in a conditional comment in the <head> section in your layout:
<!--[if lt IE 9]>
<%= javascript_include_tag 'html5shiv-printshiv' %>
<![endif]-->