Update dependencies

* I updated all the dependencies to the latest version.
* I added `files` field to slim down npm package.
* I added `before_install` script to update [npm](https://www.npmjs.com/) to the latest version.
  * The version of npm bundled with Node 0.8 is v1.2.30. npm v1.2 doesn't support the ^ version specifier and [it causes an error when installing](https://travis-ci.org/jakubpawlowicz/clean-css/jobs/26472936).
Thanks for explanation! You needn't to fix that.