Updated string_cache to 0.2, and updated the dependencies that depend on string_cache.
Removed references to string_cache_plugin.
Import atom! and ns! from string_cache.
Replaced ns!("") by ns!().
Replaced ns!(XML) and co by ns!(xml) and co.
Replaced Atom::from_slice by Atom::from.
Replaced atom.as_slice() by &*atom.
r? @SimonSapin
Source-Repo: https://github.com/servo/servo
Source-Revision: 188fa9378c103093f1f8dac24bff0d9d237fd2bc