chore: add [email protected] as a dev dependency

This is mainly to make builds on Node.js 6.x work correctly. The latest version of `semantic-release` pulls in `[email protected]` as a dependency, which is not compatible with Node.js 6.x. We don't really call `semantic-release` on Node.js 6.x, but this dependency causes `[email protected]` to be used when calling `npm` with Node.js 6.x, which causes all `npm` command executions to break.