Update to current rust: use the new attributes syntax.
Update to current rust: use the new attributes syntax.

Builds with mozilla/rust@e263ef1df7b892ed29e53313565eb05ab75e52f4.

tempbottle/rust-cssparser,frewsxcv/rust-cssparser,notriddle/rust-cssparser,servo/rust-cssparser,notriddle/rust-cssparser,frewsxcv/rust-cssparser,disjukr/rust-cssparser,disjukr/rust-cssparser,tempbottle/rust-cssparser,servo/rust-cssparser