Update vue-i18n requirement from 8.7.0 to 8.8.0

Updates the requirements on vue-i18n to permit the latest version.

Release notes

Sourced from vue-i18n's releases.

v8.8.0

v8.8.0 (2019-01-29)

🐛 Bug Fixes

• index: fix flat path based key issue (bed9c39), closes #349
• mixin: fix beforeDestroy can not find this.$t (#500) by [masongzhi](https://github.com/masongzhi) (311b8f3), closes #500

⚡ Improvements

• directive: Fix typo on warning message (#509) by [kimuraz](https://github.com/kimuraz) (e879024), closes #509
• index: silence fallback warnings (#510) by [SzNagyMisu](https://github.com/SzNagyMisu) (ddc0c79), closes #510 #139

Changelog

Sourced from vue-i18n's changelog.

8.8.0 (2019-01-29)

🐛 Bug Fixes

• index: fix flat path based key issue (bed9c39), closes #349
• mixin: fix beforeDestroy can not find this.$t (#500) by [masongzhi](https://github.com/masongzhi) (311b8f3), closes #500

⚡ Improvements

• directive: Fix typo on warning message (#509) by [kimuraz](https://github.com/kimuraz) (e879024), closes #509
• index: silence fallback warnings (#510) by [SzNagyMisu](https://github.com/SzNagyMisu) (ddc0c79), closes #510 #139

8.7.0 (2019-01-02)

⚡ Improvements

• directive: Preserve directive content (#495) by [bponomarenko](https://github.com/bponomarenko) (c29edba), closes #495 #408 #408

8.6.0 (2018-12-25)

🐛 Bug Fixes

• pluralization: inherit pluralization rules ⚠ (#493) by [Raiondesu](https://github.com/Raiondesu) (7a23f32), closes #493

⚡ Improvements

• format: Add the path as argument to the custom formatter (#489) by [Raiondesu](https://github.com/Raiondesu) (b9437ea), closes #489 #484 #484

8.5.0 (2018-12-17)

🐛 Bug Fixes

• index: evaluate availabilities lazily (fix #477) (#483) by [gamtiq](https://github.com/gamtiq) (b66f02e), closes #477 #483

... (truncated)

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