Update .travis.yml: remove deprecated `sudo: false` and add `cache: false`
Update .travis.yml: remove deprecated `sudo: false` and add `cache: false`

`cache: false` is to keep updating { "vivliostyle": "next" } to the latest development version of vivliostyle.js.

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js