v-t directive doesn't update on setLocaleMessage #450

Hello,

there is a problem, when my API call update translations with setLocaleMessage, all translations created with d-t directive are not updated, but created with method $t are updated.

vue & vue-i18n version

2.5.17 & 8.2.1

Reproduction Link

https://jsfiddle.net/w0xrfn35/1/

Steps to reproduce

open reproduction link and click button, which update messages

What is Expected?

all translations are updated

What is actually happening?

translations from d-t remain old