Use mocha correctly

see also:
- http://blog.nodejs.org/2011/03/23/npm-1-0-global-vs-local-installation/
- http://blog.millermedeiros.com/node-js-protip-avoid-global-test-runners/
- http://programmers.stackexchange.com/questions/199102/npm-packages-json-devdependencies-vs-global-installation