Bump the minimum version of the rust toolchain we require to
build. The 1.15 release includes support for custom #[derive]
directives, letting us use the serde serialization crate without
checking in a lot of generated code.
This is primarily motivated by webrender and the audio remoting
work, and lets us drop the heavy syntex dependency.
MozReview-Commit-ID: 6IObHhouPAn
--HG--
extra : rebase_source : 4be8b148fb653a48f6df4309811ab1d8755f7edf
d9052fefa0