chore(deps): update dependency ember-cli to v3.15.2

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
ember-cli (source)	devDependencies	patch	3.15.1 -> 3.15.2

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Release Notes

ember-cli/ember-cli

### [`v3.15.2`](https://togithub.com/ember-cli/ember-cli/blob/master/CHANGELOG.md#v3152) [Compare Source](https://togithub.com/ember-cli/ember-cli/compare/v3.15.1...v3.15.2) ##### Blueprint Changes - [`ember new` diff](https://togithub.com/ember-cli/ember-new-output/compare/v3.15.1...v3.15.2) - [`ember addon` diff](https://togithub.com/ember-cli/ember-addon-output/compare/v3.15.1...v3.15.2) ##### Changelog - [#​8924](https://togithub.com/ember-cli/ember-cli/pull/8924) Fix named UMD imports [@​kellyselden](https://togithub.com/kellyselden) - [#​9015](https://togithub.com/ember-cli/ember-cli/pull/9015) Allow failure of Node 8 CI jobs. [@​rwjblue](https://togithub.com/rwjblue) - [#​9014](https://togithub.com/ember-cli/ember-cli/pull/9014) Avoid errors when `ui` is not present and a warning will be emitted. [@​tmquinn](https://togithub.com/tmquinn) Thank you to all who took the time to contribute!

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