Mac OS Travis fails on master

/usr/local/Homebrew/Library/Homebrew/brew.rb:12:in `<main>': Homebrew must be run under Ruby 2.3! You're running 2.0.0. (RuntimeError)

Hm, indeed it does.

Looks like the same issue has hit multiple other projects (https://github.com/travis-ci/travis-ci/issues/8552, see in particular comment https://github.com/travis-ci/travis-ci/issues/8552#issuecomment-335430374 which explains the underlying issue). Apparently brew tries to autoupdate when installing packages and run into the error messages. The workaround for now is to update brew manually before installing packages, I'll soon push a branch where we can test this.

A proper fix might come with the new default osx images due soon https://blog.travis-ci.com/2017-10-16-a-new-default-os-x-image-is-coming so hopefully the workaround will be temporary...

The new default image has been delayed a bit, and is now due November 28th, see https://blog.travis-ci.com/2017-10-16-a-new-default-os-x-image-is-coming for details. I'll experiment a bit with this image, because I believe we can drop the workaround with `brew update`. I'll keep the branch open untill the new image is default though, because I need to override the currently default image (7.3). Once the new image is default, it will no longer be neccessary to override it, so we can drop the osx_image line from the yml config.

Fixed in build20-rc1