Corrects required version and make Eigen3 download work.

At least on Debian requiring version 3.1 of Eigen3 is not sufficient
according to issue #25 the minimum version is 3.1.2. This patch fixes
the required version. In addition it skips the REQUIRED option passed
to find_package(Eigen3) to make the git download work.

Fixes #25.
Merge pull request #476 from andlaus/fix_eigen3_version

Corrects required version and make Eigen3 download work.