I've just gone through that too, it seems the instructions for the html5-haml
template are legacy, for a previous Middleman version. Current Middleman doesn't come with it, and the under development Middleman-HTML5BP-HAML template cloned to ~/.middleman/html5bphaml/
and used through middleman init my_new_project --template=html5bphaml
has caused me problems, it installs an entire gemset and is dependent of older gems than the ones that were working with the provided templates.
The author says it's to be bundled with Middleman soon, so I expect the behavior of the template to be similar of the other ones when this happen.