Increase the CMAKE minimal version requirement

[weikengchen]

This is a very small change. Since emp-tool requires 2.8.12, and emp-ot requires 3.0, we can increase the CMake minimal version from 2.8.11 to 3.0.

This is to suppress a warning when running cmake .:

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

[weikengchen]

The CI has eventually passed.