catch_default_main.hpp was removed in db32550898
The example 000-CatchMain.cpp is no longer compiled, but is still
present in the examples and is still references by other example
files. Remove the file and references to it, as they are confusing.
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
