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

conanfile.py

@@ -9,11 +9,11 @@ class CatchConan(ConanFile):
     author = "philsquared"
     generators = "cmake"
     exports_sources = "single_include/*"
-    url = "https://github.com/philsquared/Catch"
+    url = "https://github.com/catchorg/Catch2"
     license = "Boost Software License - Version 1.0. http://www.boost.org/LICENSE_1_0.txt"
 
     def package(self):
-        self.copy(pattern="catch.hpp", src="single_include", dst="include")
+        self.copy(pattern="catch.hpp", src="single_include/catch2", dst="include/catch2")
 
     def package_id(self):
             self.info.header_only()