install valgrind on mac mojave code example

Example: checking for the kernel version... unsupported (18.7.0) configure: error: Valgrind works on Darwin 10.x, 11.x, 12.x, 13.x, 14.x, 15.x, 16.x and 17.x (Mac OS X 10.6/7/8/9/10/11 and macOS 10.12/13)

brew install --HEAD https://raw.githubusercontent.com/sowson/valgrind/master/valgrind.rb