Auto merge of #5083 - matklad:Cargo-the-tool-1.0, r=alexcrichton

Auto merge of #5083 - matklad:Cargo-the-tool-1.0, r=alexcrichton

Make cargo-the-binary version the same as the Rust version

Closes #4211 which seems stuck :-)

Soooo, this is the simplest possible fix to this problem:

```
~/projects/cargo Cargo-the-tool-1.0
λ rustc --version
rustc 1.24.0 (4d90ac38c 2018-02-12)

~/projects/cargo Cargo-the-tool-1.0
λ cargo --version
cargo 0.25.0 (8c93e0895 2018-02-01)
```

It makes `cargo --version` to print the corresponding Rust version (with possibly cargo-specific patch number), while keeping the library version the same as today.

On the one hand, this is horrible. On the other hand, it seems to do the job? In the long term, it would be cool skip one version bump for the library, so that `0.x` corresponds to `1.x`.

cc @rust-lang/cargo

I am not sure that this is good idea, the implementation certainly feels horrible :)