@musicmatze we had the same problem. We ended up using the latest ubuntu image and are now setting up rustup manually in the ci configuration. Afaik travis is still using xenial for rust builds (check the build log) and setting dist to bionic is ignored. Set `language: minimal` and `dist: bionic` to work around the issue.
@musicmatze note: travis is not shipping tmux-2.1 in their bionic image, but rather ignores to use the bionic image and instead is using xenial. I was not aware of that when writing the mentioned toot.
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