Auto merge of #57721 - ehuss:update-cargo, r=alexcrichton

Update cargo

Unblocks #56884  cc @euclio

6 commits in 2b4a5f1f0bb6e13759e88ea9512527b0beba154f..ffe65875fd05018599ad07e7389e99050c7915be
2019-01-12 04:13:12 +0000 to 2019-01-17 23:57:50 +0000
- Better error message for bad manifest with `cargo install`. (rust-lang/cargo#6560)
- relax rustdoc output assertion (rust-lang/cargo#6559)
- touch some files when we use them (rust-lang/cargo#6477)
- Add documentation for new package/publish feature flags. (rust-lang/cargo#6553)
- Update chat link to Discord. (rust-lang/cargo#6554)
- Fix typo (rust-lang/cargo#6552)

r? @alexcrichton