Update cmake minimum version

[devintrowbridge]

Running cmake gives the following warning

CMake Deprecation Warning at CMakeLists.txt:5 (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.

I'd like to submit a PR to update from 2.8.8 to 2.8.12 like the warning suggests. Wanted to see if there would be any issues with upgrading first though.