We had some issues when we tried to download the CUDA Toolkit .deb file and use the apt-get package manager to install CUDA Toolkit on Ubuntu 15.04. So instead we just manually installed the CUDA Toolkit .run file.
We used these resources to piece together the right steps for doing this. http://developer.download.nvidia.com/compute/cuda/7.5/Prod/docs/sidebar/CUDA_Installation_Guide_Linux.pdf http://www.allaboutlinux.eu/remove-nouveau-and-install-nvidia-driver-in-ubuntu-15-04/2/ https://askubuntu.com/questions/16371/how-do-i-disable-x-at-boot-time-so-that-the-system-boots-in-text-mode
Remove existing nvidia stuff:
sudo apt-get remove nvidia*