use space, not dash, in `cargo --version` output

use space, not dash, in `cargo --version` output

 #3604 inadvertently changed the output format of `cargo --version`:

```
froydnj@hawkeye:~/src/cargo.git$ ./target/x86_64-unknown-linux-gnu/release/cargo --version
cargo-0.17.0-dev (ae4a4d8 2017-01-27)
froydnj@hawkeye:~/src/cargo.git$ cargo --version
cargo 0.16.0-nightly (6e0c18c 2017-01-27)
```

Note the dash between cargo and the version number in the newer version.

Let's put the space back to not break things that might depend on the
`--version` output.