opm-core patch syncing opm-autodiff patch concerning Eigen version with all modules

This is the change that was already aplied in autodiff while merging pull
request #77. For syncing the modules it has to applied to opm-autodiff and
the rest of the modules using @rolk magic.

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.