This was an old "include all" header, that we no longer want to be
usable, to make the include differences in new versions explicit.
We will introduce new "include all" headers later, in the form of
`catch_all.hpp`, `catch_matchers_all.hpp` and so on...
This also goes for pkg-config installed by our CMake installation.
This includes
* Updating CMake version on Travis
* Adding a `Catch2` subfolder to the `single_include/` folder to
provide this include path both _inside_ the repository, and _outside_.
* Updated examples to build with the new paths
* Other general CMake cleanup