v0.2.0: update to latest keyring-core

This crate now relies on keyring-core v0.7.0.

What's Changed

• Update to keyring-core v0.7 by @brotskydotcom in #4
• Bump version to v0.2.0 by @brotskydotcom in #5

Full Changelog: v0.1.0...v0.2.0