Have you installed the phantomjs and casperjs runtime? The npm packages for phantomjs and casperjs call out to these runtimes. The error you see is when it can not find the casperjs runtime.

I'm not very familiar with nitrous.io and how much freedom you have to install other runtimes. Here's what I used for installing them on Linux: https://www.dvdheiden.nl/2013/07/30/installing-phantomjs-and-casperjs-on-linux/