= javascript_include_tag "handlebars.runtime"
If you put this at the end of your view it will be included in the document at that point. If you want it to be included in the head or at the bottom use the yield <symbol>
functionality to achieve that. for instance if I have a <%= yield :head %>
in my layout I can put the include tag call above into a content_for:
= content_for :head do
= javascript_include_tag "handlebars.runtime"