Update Travis to use faster cintainer-based (Docker) infrastructure

Travis offers a [new, faster infrastructure](http://docs.travis-ci.com/user/workers/container-based-infrastructure/) that uses Docker. The only downside is that it doesn't allow for `sudo` access. 

This PR cleans up the dist.ini (we have duplicate commands in the dist and travis.yml file) and updates travis.yml to install our only system dependency, GNU MPFR, via a shell script, to avoid the use of `sudo`

Travis also allows for caching of folders and packages, we could potentially cache the CPAN packages but I'm not sure about that