cmake: Bump required version

cmake: Bump required version

Using 'project(foo VERSION 1.0)' requires CMake 3.0, the VERSION option
is not part of CMake 2.8.7, see:

* https://cmake.org/cmake/help/v2.8.7/cmake.html#command:project
* https://cmake.org/cmake/help/v3.0/release/3.0.0.html#commands

The property C_STANDARD was introduced with CMake 3.1, see:

* https://cmake.org/cmake/help/v3.1/release/3.1.0.html#properties

Signed-off-by: Alexander Dahl <post@lespocky.de>