Fix travis (again)

[Cobrand]

Looks like there is an issue with rustup on nightly: https://travis-ci.org/AngryLawyer/rust-sdl2/jobs/221621182

[Xaeroxe]

Nightly is definitely prone to random failure, and provides zero stability guarantees. I recommend leaving nightly as an "allowed failure" in the .travis.yml. I can make a PR to do this if you want.

[Cobrand]

Except it's not nightly itself that fails, it's the download of nightly via rustup.rs . My guess is that either it was a mistake on their end, or they deprecated "nigtly" as a target and thus it doesn't work anymore.

Precisely, this fails :
curl -sSf https://sh.rustup.rs | sh -s -- --default-toolchain=$TRAVIS_RUST_VERSION -y

And we definitely want to adapt to nightly as well. Too many users develop on nightly to ignore this. Random failures are fine, we can run travis once more, but this looks like something became deprecated on rustup without our knowledge.

[Xaeroxe]

Personally I'm of the opinion that trying to support nightly users is an exercise in futility, but this also isn't my repository. This also pretty clearly looks like an upstream problem to me, and I can't find anything from Rustup saying they no longer support nightly, so perhaps an issue should be filed upstream with them?

[Cobrand]

Travis is back ! We just had to ask nicely.

[Cobrand]

rust-lang/rustup#1062