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

test_package/MainTest.cpp

@@ -6,7 +6,7 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 #define CATCH_CONFIG_MAIN
-#include "catch.hpp"
+#include <catch2/catch.hpp>
 
 unsigned int Factorial( unsigned int number ) {
   return number > 1 ? Factorial(number-1)*number : 1;