servo: Merge #18325 - Switch back to pinning Rust by Nightly date ins…

…tead of commit hash… (from servo:rustup-toolchain); r=nox

… this time using a `rust-toolchain` file compatible with rustup: https://github.com/rust-lang-nursery/rustup.rs/#the-toolchain-file

And upgrade to rustc 1.21.0-nightly (c11f689d2 2017-08-29)

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Now if both `system-rust` and `system-cargo` are set to `true` in `.servobuild`’s `[tools]` section, and the corresponding `rustc` and `cargo` binaries are in fact rustup’s wrappers, then rustup will use the correct version based on `rust-toolchain`.

CC servo/servo#11361

Unlike servo/servo#17927, this does not make mach use rustup directly. That should wait until rust-lang/rustup#1099 is fixed.

Source-Repo: https://github.com/servo/servo
Source-Revision: c4800a6c83e6fdabaf7c4eff70a24487d16f18ff

UltraBlame original commit: 5df6f8fe6c50631fe00f47b69d16b8d555d90503

servo/README.md

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servo/appveyor.yml

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