CI: Change back to stable toolchain

[tesuji]

PR #827 changed default CI toolchain to beta to sort out cargo warning
for cargo feature default-run. We should revert it back when we done
dealing with latest rustc.

[RalfJung]

We should revert it back when we done dealing with latest rustc.

Correction: we can only revert it back once Rust 1.37 hits stable.

[RalfJung]

Actually, thinking about it, using the beta compiler seems like a nice idea. That's very close to the version rustc uses for bootstrapping, which is good -- that's the cargo version used in rustc CI.

[tesuji]

This could be closed now.