Add coffee-script@1.6.3 to devDependencies

`npm test` does not work.

```
$ npm test

> hubot-hello-world@0.0.0 test /home/bouzuya/hubot-hello-world
> grunt test

Running "mochaTest:test" (mochaTest) task
Warning: Cannot find module 'coffee-script' Use --force to continue.

Aborted due to warnings.
npm ERR! Test failed.  See above for more details.
npm ERR! not ok code 0
```