Auto merge of #5912 - rust-lang:dependabot/cargo/opener-0.3.0, r=alexcrichton

Auto merge of #5912 - rust-lang:dependabot/cargo/opener-0.3.0, r=alexcrichton

Update opener requirement from 0.2.0 to 0.3.0

Updates the requirements on [opener](https://github.com/Seeker14491/opener) to permit the latest version.
<details>
<summary>Changelog</summary>

*Sourced from [opener's changelog](https://github.com/Seeker14491/opener/blob/master/CHANGELOG.md).*

> ## [0.3.0] - 2018-08-18
> ### Added
> - `stderr` field to `OpenError::ExitStatus` variant, which captures anything the failed process wrote to stderr.
>
> ## [0.2.0] - 2018-08-08
> ### Removed
> - The `open_sys` function, which was erroneously pub on non-Windows builds.
>
> ## [0.1.0] - 2018-08-08
> - Initial release.
</details>
<details>
<summary>Commits</summary>

- See full diff in [compare view](https://github.com/Seeker14491/opener/commits/v0.3.0)
</details>
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