fix cmake version for tests

[leongross]

When compiling the project an error is thrown:

CMake Error at tests/CMakeLists.txt:48 (target_sources):
  Unknown CMake command "target_sources".

This is due to the cmake option target_sources introduced in CMake version 3.1.
I suggest to add the minimum required version in the CMake file for the test.
Also I added a notion in the main README.md

[coveralls]

Coverage remained the same at 87.444% when pulling 8be2ffd on leongross:fix/test-cmake3.1 into 4fc44f3 on json-c:master.