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](vuejs/vue@v2.5.20...v2.5.21)

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

package.json

@@ -37,7 +37,7 @@
     "relative-date": "^1.1.3",
     "socket.io": "^2.2.0",
     "socket.io-client": "^2.2.0",
-    "vue": "^2.5.20",
+    "vue": "^2.5.21",
     "vueify": "^9.4.1",
     "watchify": "^3.11.0"
   },