Remove rust channel

[Arzte]

This is done to prevent a default build from spawning, (it looks weird, its better to have the build as part of the matrix.)
Also the default is to run on stable anyways so its even more silly.

[japaric]

Thanks for the PR, @SirDoctors.

This is done to prevent a default build from spawning

Yay!

Also the default is to run on stable

Wait, what? How come TRAVIS_RUST_VERSION works if rust: * is not used? That seems like a brittle thing to rely on. I would prefer to have something explicit like global: env: RUST_CHANNEL = stable and the leave the elements of the matrix override that if needed. That approach would also support setting nightly as the default channel. (P.S. ci/install.sh would have to be update to use the RUST_CHANNEL variable)

I think you can use language: generic to have nothing run before before_install. Because right now the builds are defaulting to a ruby environment.

That macOS failure looks odd ...

[Arzte]

Travis's default is to use the latest stable rust version, its not very brittle at all. Builds aren't defaulting to ruby, they only do that when the language isn't set/is set to generic.

As far as that one osx error it seem to be a issue with cross not being in the env after installing, I'd give that job a restart to see if it does it again.

[japaric]

Builds aren't defaulting to ruby, they only do that when the language isn't set/is set to generic.

But 'rust: stable' is not specified in most cases. Travis seems to be infering that the repo is rust somehow (from the repo contents probably). I wonder if it's assuming 'rust: stable' because some job in the matrix is 'rust: nightly'.

As I mentioned above, this PR makes the use case of "default to rust: nightly" unergonomic because you'll have to specify 'rust: nightly' in every build.

I still would prefer the 'language: generic' + a 'RUST_CHANNEL' env variable approach.

[Arzte]

Its not, it's inferring that its rust stable from language: rust in the top of the file. When language: rust is set, Travis defaults to rust: stable for all builds that don't specify otherwise.

[japaric]

Ohh, I totally forgot that was there.

OK. This seems fine to land as it is.

I'll retry the build to see what's up with the macOS error.

[japaric]

It's green this time.