"error: toolchain 'beta' is not installed" on Travis

https://travis-ci.org/oconnor663/shared_child.rs/jobs/408676715

My `.travis.yml` is:

```
language: rust
os:
  - linux
  - osx
rust:
  - stable
  - beta
  - nightly
```

Of those 6 builds, only the OSX beta is failing, with `error: toolchain 'beta' is not installed`. Is this an upstream toolchain issue?