justshipit I've had success, but with another cartridge.
The one I used was https://github.com/boekkooi/openshift-diy-nginx-php
The only problem is that you have to wait like an hour for the build to finish, but I think it actually works.
2nd thing to keep in mind is that you first need to
1. Download Flarum
2. Install Flarum
And then, after finishing instructions on the cartdrige's README:
3. Move Flarum contents, aka vendor/, config.php... into the web folder inside cloned repository, making sure you are moving the contents of the forum that you've already installed in step 2.
EDIT 3/11/16: For a silly reason I removed the Openshift app from openshift & locally, and I have failed to put it up working again ?