JAPR neither concatenating stylesheets nor producing error

[rhewitt22]

I followed along with the instructions, downloaded the gem and included a jekyll_asset_pipeline.rb file in my plugins directory that requires the gem. I also tried using the _config.yml gem option to include the plugin.

I have two CSS files that aught to be concatenated, but the resultant html file in _site contains the same liquid tags as the input with no error:

{% css_asset_tag global %}
- /_assets/test.css
- /_assets/testing.css
{% endcss_asset_tag %}

I have another plugin running so I know that I'm not in safe mode. I'm using Jekyll version 2.4.0.