The use we previously used the polyfill or naked new is that we
supported C++11, which did not yet have `std::make_unique`. However,
with the move to C++14 as the minimum, `std::make_unique` can be
expected to be always available.
* Use Xenial as the base distribution
* Remove C++11 builds
* Add a lot more C++14 builds
* Add some C++17 builds
* Include newer versions of Clang and GCC
