Explicitly install gcc72-c++
Explicitly install gcc72-c++

Fixes #182
Fixes #183
Seems it's not part of the `development` group, whereas gcc72-gcc is 🙄

lambci/docker-lambda,lambci/docker-lambda,lambci/docker-lambda,lambci/docker-lambda,lambci/docker-lambda,lambci/docker-lambda,lambci/docker-lambda