Upgrade: Bump vuepress from 2.0.0-beta.6 to 2.0.0-beta.8

Bumps [vuepress](https://github.com/vuejs/vuepress/tree/HEAD/packages/vuepress) from 2.0.0-beta.6 to 2.0.0-beta.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)

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

package.json

@@ -13,7 +13,7 @@
     "sass": "^1.32.8",
     "sass-loader": "^11.0.1",
     "vue-property-decorator": "^9.1.2",
-    "vuepress": "^2.0.0-beta.6"
+    "vuepress": "^2.0.0-beta.8"
   },
   "scripts": {
     "docs:dev": "vuepress dev docs",