[CI] [Rust] New 1.40.0 nightly causing builds to fail

So much for pinning the nightly version ... that doesn't work when there is a new major version of a nightly apparently.

Travis is now using:

rustc 1.40.0-nightly (37538aa13 2019-09-25) 

Despite rust-toolchain containing:

nightly-2019-07-30