cmake minimum version

[gonsie]

I just noticed this message:

CMake Deprecation Warning at CMakeLists.txt:2 (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.

We should probably consider upping to v3.0 along with tracking CMake's deprecation of 2.8 compatibility.