Bump vue from 2.5.20 to 2.5.21

Bumps [vue](https://github.com/vuejs/vue) from 2.5.20 to 2.5.21.
- [Release notes](https://github.com/vuejs/vue/releases)
- [Commits](https://github.com/vuejs/vue/compare/v2.5.20...v2.5.21

)

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