Auto merge of #6761 - alexcrichton:handle-toml, r=ehuss
Auto merge of #6761 - alexcrichton:handle-toml, r=ehuss

Handle backcompat hazard with `toml` crate

The 0.5.0 release contained a
[bugfix](https://github.com/alexcrichton/toml-rs/pull/280) which
previously successfully parsed invalid TOML files, so we'll need to
manually handle that in Cago to ensure that we don't accidentally
regress crates, but rather instead give them an appropriate amount of
time to get fixed.

rustyhorde/cargo,vojtechkral/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,nipunn1313/cargo,eminence/cargo,tsurai/cargo,rustyhorde/cargo,tsurai/cargo,vojtechkral/cargo,eminence/cargo,rust-lang/cargo,tshepang/cargo,vojtechkral/cargo,rust-lang/cargo,vojtechkral/cargo,tsurai/cargo,rust-lang/cargo,tsurai/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,rustyhorde/cargo,eminence/cargo,tshepang/cargo