How to reference ActionText::RichText from outside Rails app?

For testing the integration of the Mobility translation library with ActionText rich text, there would need to be a way to load the ActionText::RichText class even if not in the context of a Rails app. However simply referencing the class yields the error uninitialized constant ActionText. Maybe it’s because of some missing autoloading mechanism? Is there any way to require the class (tried adding require "action_text/rich_text" to no avail)?