WIP: Switch travis-ci build to 64 bit ROOT binaries

Travis recently switched the Python VMs from 32 bit to 64 bit:
https://groups.google.com/d/msg/travis-ci/6XGJ6Jw628k/ppujf6zNLs0J
causing the `rootpy` build to fail.

Here we switch to 64 bit ROOT binaries to make it work again.