CMake minimum required version should actually be 3.1

[jneuff]

Release/CMakeLists.txt says cmake_minimum_required(VERSION 3.0).
But Release/src/CMakeLists.txt makes use of the target_sources command which is only available
in CMake 3.1 and above.

[wtfrank]

I also experienced this issue, with cpprest 2.10.1.