Cleanup some code in image_cache_task.rs.
auto merge of #3815 : Ms2ger/servo/image-cache-task, r=glennw
Fix cargo.lock that got out of sync when two successive commits (1) updated the SHA (2) added the dependency to util with the old SHA.
auto merge of #3816 : glennw/servo/cargo-lock-fix, r=Manishearth