Ignore `grunt-cli` dependency when loading all grunt tasks

This removes the `Local Npm module "grunt-cli" not found. Is it
installed?`
[message](https://travis-ci.org/cerebris/orbit.js/builds/16777892#L770)
when executing `npm test` on Travis.

This is inspired by http://git.io/SVuF9w
Merge pull request #4 from pangratz/ignore_grunt-cli_when_loading_tasks

Ignore `grunt-cli` dependency when loading all grunt tasks