🔖 set version to 3.1.0

- updated documentation wrt. new repository layout
- temporarily switched off Homebrew --HEAD building (can only be switched on after release)
- set copyright date to 2018

doc/examples/rbegin.cpp

@@ -1,5 +1,5 @@
 #include <iostream>
-#include "json.hpp"
+#include <nlohmann/json.hpp>
 
 using json = nlohmann::json;