update dependencies
add `files` field to slim down npm package

https://docs.npmjs.com/files/package.json#files
simplify test script

https://docs.npmjs.com/misc/scripts#path
update npm before installing dependencies

Because 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