Tryout - Add EVM (emacs version manager) to deal with emacs versions on travis

*Note* Tryout because I need to make it work first and for this I need
 to open the PR...

Again, it's a proposal (this eases the .travis.yml's readability).

Also, this reduces the use of [Damien Cassou's PPA which is no longer
maintained](https://launchpad.net/~cassou/+archive/ubuntu/emacs).

Cheers,