This answer is a bit orthogonal to the the OP, but --
libsass bindings don't install properly with the node-sass wrapper on Node v4.0.0. I got the same error message as in the question (Error: 'libsass' bindings not found. Try reinstalling 'node-sass')
but I ended up uninstalling Node v4.0.0 and installing v0.12.7 using nvm, via this script:
https://gist.github.com/brock/5b1b70590e1171c4ab54
and now libsass and node-sass are behaving properly.