Bump vuepress from 2.0.0-rc.7 to 2.0.0-rc.8

Bumps [vuepress](https://github.com/vuejs/vuepress/tree/HEAD/packages/vuepress) from 2.0.0-rc.7 to 2.0.0-rc.8.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/HEAD/packages/vuepress)

---
updated-dependencies:
- dependency-name: vuepress
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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