Add Cypress v15 support to peerDependencies

[jimblue]

Cypress has just released v15.0.0.

To ensure compatibility, cypress-axe should include v15 in the peerDependencies section of package.json.

[jennifer-shehane]

This PR is opened to support that: #187

[RickePicke]

@jennifer-shehane Hi! Nice work with #187, quickly done! 👌 Do you know when it will be merged and published?