Replace cmake ppa with a debian backport that is more frequently updated (and generally better)

This ppa also doesn't build from source, and names cmake properly, so we can keep our requirements as they are! (ubuntu != gentoo)

This commit has been tested on both stock 14.04 dockerimages and 14.04 dockerimages of the previous commit (and compiled soccer with no errors), so it shouldn't cause any trouble.

Removing the other ppa is not necessary as this ppa has a newer version of cmake.
This command will remove the old ppa, if you want to do that anyway:
```
sudo apt-add-repository -y --remove ppa:andykimpe/cmake3
```