update npm before installing dependencies
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

dfilatov/vow-fs,marcodejongh/vow-fs