Increase minimum CMake required version

[alextoind]

Since #6 has been fixed by using set_target_properties() on all targets (including tests), it might be better to evaluate to increase the minimum CMake required version.

This would allow to exploit the more appropriate target_compile_features(Serial PUBLIC cxx_std_11) (available since CMake 3.8) which should also export the feature to all the targets which link to the library, e.g. the tests.

Moreover, this would also let the compiler to choose higher language standard and not only the specified version, cf. https://crascit.com/2015/03/28/enabling-cxx11-in-cmake.