Someone made a good writeup for the installation process:
http://www.tylerforsythe.com/2015/04/install-redmine-onto-raspberry-pi-2-this-is-the-tutorial-you-want/
and
http://www.tylerforsythe.com/2015/04/redmine-version-3-onto-raspberry-pi-2/
Curious how the performance is going to be, for a small team this could work I guess.