The new pybind11 bindings require at least Eigen-3.2.7. Travis and Ubuntu 14.04 only include Eigen-3.2.2, so let's include Eigen as a submodule.
Unfortunately Eigen uses Mercurial and no official git mirror exists, so I've cloned Eigen's mercurial repo and added it at patrikhuber/eigen.git at version tag 3.3.3.