Auto merge of #3338 - alexcrichton:correct-vers-flag, r=brson
authorbors <bors@rust-lang.org>
Fri, 2 Dec 2016 13:23:38 +0000 (13:23 +0000)
committerbors <bors@rust-lang.org>
Fri, 2 Dec 2016 13:23:38 +0000 (13:23 +0000)
Require `cargo install --vers` takes a semver version

Historically Cargo accidentally took a semver version *requirement*, so let's
start issuing warnings about how this is now legacy behavior.

Closes #3321


Trivial merge