I’m aware that Kwant is not yet available in Ubuntu 15.04. It’s on my list of things to do, but frankly I have been too busy the last weeks to take care of it. I hope to have time next week to deal with it.
For now you can try to install Kwant from source. This is very easy on an Ubuntu system, please see the installation instructions. It’s possible that the problems that you see are not only due to packaging but also due to incompatibilities with newer libraries. In that case you’ll have to wait or fix the issues yourself.