Cephes requires Cmake 3.12, breaks build on Ubuntu Bionic

[berndpfrommer]

The nightly Ubuntu Bionic builds break because the newly-added cephes library requires CMake 3.12 or higher

-- Found Eigen version: 3.3.4
CMake Error at gtsam/3rdparty/cephes/CMakeLists.txt:1 (cmake_minimum_required):
  CMake 3.12 or higher is required.  You are running version 3.10.2

Options:

1. Drop support for Ubuntu Bionic
2. Change required cmake version for libcephes to 3.10. I can build it locally on my machine without encountering errors. It is likely that libcephes does not rely on any features introduced between 3.10 and 3.12

[varunagrawal]

Fixed by #1711. Closing, thanks @berndpfrommer