Bump vuex from 3.1.1 to 3.1.2

Bumps [vuex](https://github.com/vuejs/vuex) from 3.1.1 to 3.1.2.
- [Release notes](https://github.com/vuejs/vuex/releases)
- [Commits](https://github.com/vuejs/vuex/compare/v3.1.1...v3.1.2

)

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