Resolve CI failures for beta-x86_64-unknown-linux-gnu

[Susurrus]

I've just run cargo test using both the beta and nightly toolchains, and neither failed. I'm guessing this is a CI failure caused by caching. With beta failing, it's not doing us any good because we always ignore it. I suggest we work to resolve this failure and change our CI tests so that failures running the beta toolchain will fail the whole build.

[asomers]

It passes for me, too. But I just cleared Travis's caches and restarted a recent build, and it failed again :( . Any other ideas?

[Susurrus]

I'm wondering if it's reusing a cache from a previous Travis build from the same run, not one across runs. Though I don't really get why the caching causes problems in the first place either.

[Susurrus]

For example, I wonder if running tests only on that platform would work.

[Susurrus]

Resolved with #805.