Install Ubuntu on Yosemite
This may not be the answer you are looking for but I thought I'd boot it out there just in case. Assuming you have sufficient RAM (6 - 8GB) it might be easier to install Ubuntu (or any Linux dist.) in a virtual Machine. Parallels and VMWare Fusion are the two big commercial virtualization apps, but I have found that the free (and frequently updated) VirtualBox is easier than doing a dual boot system with the added benefits of it being quite easy to have both running at the same time.

As virtualbox is free you could try it and see if it suits your needs...