Note that 1.10.0 is required

Turns out we're relying on a bugfix in 1.10.0 which renders 1.9.0, the first
release with `catch_unwind`, not usable.

Closes rust-lang#59

TUTORIAL.md

@@ -43,7 +43,7 @@ If you'd like to help contribute to this document you can find it on
 
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-The `futures` crate requires Rust 1.9.0 or greater, which can be easily
+The `futures` crate requires Rust 1.10.0 or greater, which can be easily
 obtained through [rustup]. Windows, macOS, and Linux are all tested and known to
 work, but PRs for other platforms are always welcome! You can add
 futures to your project's `Cargo.toml` like so: