Ensure Eigen is at least 3.1.0 for Eigen/SparseCore.

- Eigen/SparseCore is required by covariance_impl, this was added in
  v 3.1.0 of Eigen, and thus without at least this version Ceres will
  not compile.
- Note that Ubuntu 12.04 provides only version 3.0.5 in the mainline
  repository.
- Update docs to match CMake check for Eigen >= 3.2.2 to avoid warning
  about reduced sparse performance.

Change-Id: I291bb185d1c76e1e1422429169a76e3f1b828163