`npm install` in racer should work. Make sure you're in
node_modules/derby/node_modules/racer. The reason is mocha is installed as
a devDependency, not a dependency (package.json) - and those are only
installed when you're directly in that folder running `npm install`.
devDependencies are for when you're actually developing that module, so you
need test modules (mocha & expect.js), compilers (coffeescript), etc.
Derby's Makefile just piggy-backs off Racer's devDependencies - if that's
still giving you hell, you can just replace
"./node_modules/racer/node_modules/mocha/bin/mocha" in Derby's package.json
to "./node_modules/mocha/bin/mocha"