Remove Ruby references

After changing from Ruby Sass to libsass, issue [#82](https://github.com/google/web-starter-kit/issues/82) / Commit [5b860b2](https://github.com/google/web-starter-kit/commit/5b860b2945376957663aa2f11b47b0c6a1387cdb) .  A note: I only changed what I could read in /_zh-cn.