`brew install` fails for non-Objective-C builds

Environments macOS
1

This used to work a month ago:

https://travis-ci.org/acgetchell/CDT-plusplus/jobs/470195928

It fails as of 2 days ago:

https://travis-ci.org/acgetchell/CDT-plusplus/jobs/477401945#L1198

2

Homebrew 1.9.x introduced this breaking change. See https://twitter.com/travisci/status/1083841875646263296

3

I’ve reported this issue upstream as https://github.com/Homebrew/brew/issues/5524. For Objective-C builds, we prevent automatic Homebrew updates, so they are not affected by this change.

4

This is now fixed by Homebrew 1.9.2.

(We may extend HOMEBREW_NO_AUTO_UPDATE=1 to all Mac builds to prevent this kind of issues in the future.)