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).
Because YAML parses it as a number `0.1`, not a string `0.10`.

![image](https://cloud.githubusercontent.com/assets/1131567/5515812/5bb71ac2-88bf-11e4-8f79-d0966f1e74fe.png)

Travis CI automatically regards it as Node v0.10 for safety, but I added quote to follow YAML specification.

Though, If you don't prefer it, I'll fix this PR soon.