CMakeLists.txt: increase cmake_minimum_required version

[jmillan]

Compatibility with CMake < 3.5 has been removed from CMake.

This would require a new libsdptransform version. Dependant projects are now forced to use the flag:
-DCMAKE_POLICY_VERSION_MINIMUM=3.5:

This is the error given by CMake when trying to build the project:

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.