cannot resolve path when there are no .coffee files on completed koans folder

[waghcwb]

To reproduce this error you should

1. Clone coffescript-koans repo (git clone git@github.com:sleepyfox/coffeescript-koans.git)
2. Run npm install
3. Run npm test

If there are no .coffee files on completed-koans folder, we get this error

> coffeescript-koans@1.2.0 test /home/your-username/your-folder/coffeescript-koans
> mocha --compilers coffee:coffee-script/register -R spec completed-koans/*.coffee

/home/your-username/your-folder/coffeescript-koans/node_modules/mocha/lib/utils.js:634
        throw new Error("cannot resolve path (or pattern) '" + path + "'");
        ^

Error: cannot resolve path (or pattern) 'completed-koans/*.coffee'
    at Object.lookupFiles (/home/your-username/your-folder/coffeescript-koans/node_modules/mocha/lib/utils.js:634:15)
    at /home/your-username/your-folder/coffeescript-koans/node_modules/mocha/bin/_mocha:327:30
    at Array.forEach (native)
    at Object.<anonymous> (/home/your-username/your-folder/coffeescript-koans/node_modules/mocha/bin/_mocha:326:6)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:390:7)
    at startup (bootstrap_node.js:150:9)
    at bootstrap_node.js:505:3

npm ERR! Test failed.  See above for more details.