Upgrade to newer version of binutils for nightly + musl

[shepmaster]

As mentioned in rust-lang/rust#34978, the Rust-provided MUSL components are now being built with a newer version of binutils, one that is incompatible with the version provided in the current container. The current solution is to upgrade binutils in some fashion. For Ubuntu-based containers, upgrading to 16.04 contains the fixes. I'm not sure what version of Debian will include them.

An example of output where the versions mismatch.

Thanks for all your hard work!

[liuchong]

rust-lang-deprecated/rust-buildbot#110
The above link said the "newer RELX relocation optimizations" been turn off?

😹 Anyway, let's upgrade debian version to "stretch" to solve this problem:

as -version
GNU assembler (GNU Binutils for Debian) 2.26.1
...

[shepmaster]

Thank you very much! I've got my code compiling in Travis now!