Integrate CMake with <catch2/catch.hpp> include paths

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

examples/020-TestCase-2.cpp

@@ -2,7 +2,7 @@
 
 // main() provided by Catch in file 020-TestCase-1.cpp.
 
-#include "catch.hpp"
+#include <catch2/catch.hpp>
 
 int Factorial( int number ) {
    return number <= 1 ? number : Factorial( number - 1 ) * number;  // fail