At some point we moved over to catch2:catchorg (notice lowercase `c`)
instead of Catch2:catchorg, but we kept uploading the released
packages to the upper-cased repository... Time to fix this, and then
merge them again.
* 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