Bump rustversion from 1.0.14 to 1.0.15 (#19)

Bumps [rustversion](https://github.com/dtolnay/rustversion) from 1.0.14 to 1.0.15.
- [Release notes](https://github.com/dtolnay/rustversion/releases)
- [Commits](dtolnay/rustversion@1.0.14...1.0.15)

---
updated-dependencies:
- dependency-name: rustversion
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]