TravisCI: Add libmpfr-dev to the dependencies.

Added `libmpfr-dev` to the dependencies because https://metacpan.org/pod/DateTime::Astro is needed by a few IAs that are about to get merged. I used the instructions from http://docs.travis-ci.com/user/installing-dependencies/#Installing-Ubuntu-packages

@kablamo @mwmiller @mintsoft @malbin do you guys think we should still use the [`[TravisCI]`](https://metacpan.org/release/Dist-Zilla-TravisCI) plugin just for generating `.travis.yml`? It seems like overkill (not to mention two files get updated each time), but that's just me. What do you think?