Bump ember-cli from 3.14.0 to 3.15.1

Bumps [ember-cli](https://github.com/ember-cli/ember-cli) from 3.14.0 to 3.15.1.
- [Release notes](https://github.com/ember-cli/ember-cli/releases)
- [Changelog](https://github.com/ember-cli/ember-cli/blob/v3.15.1/CHANGELOG.md)
- [Commits](ember-cli/ember-cli@v3.14.0...v3.15.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>