Run travis on rustc master instead of nightlies

[oli-obk]

No description provided.

[oli-obk]

@kennytm rustup-toolchain-install-master keeps randomly failing in our CI in the middle of downloading:

downloading <https://s3-us-west-1.amazonaws.com/rust-lang-ci2/rustc-builds/15add366faac554db51244d1e27e4d10a08ab6e8/rustc-nightly-x86_64-unknown-linux-gnu.tar.xz>...
23.66 MB / 71.84 MB [===========>------------------------] 32.93 % 5.65 MB/s 9s thread 'main' panicked at 'flush() fail', 

Any suggestions other than putting it in a loop until it succeeds?

[kennytm]

@oli-obk I'll take a look. flush() fail? 🤔

[phansch]

Just noting that there's travis_retry, so on Travis we would not have to implement the loop ourselves.

[phansch]

Looks like it fails even with the retries: https://travis-ci.org/rust-lang-nursery/rust-clippy/builds/376756827#L735-L747

[kennytm]

I've published rustup-toolchain-install-master 0.2.0 which should have fixed the issue.

[Manishearth]

Moved over to #2940, the merge conflict here is weird because of the first commit (?)