chore(deps): bump rollup and vuepress

Removes [rollup](https://github.com/rollup/rollup). It's no longer used after updating ancestor dependency [vuepress](https://github.com/vuejs/vuepress/tree/HEAD/packages/vuepress). These dependencies need to be updated together.


Removes `rollup`

Updates `vuepress` from 2.0.0-beta.51 to 2.0.0-rc.15
- [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: rollup
  dependency-type: indirect
- dependency-name: vuepress
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>