Bump vue-template-compiler

Bumps [vue-template-compiler](https://github.com/vuejs/vue) from 2.5.20 to 2.5.21.
- [Release notes](https://github.com/vuejs/vue/releases)
- [Commits](https://github.com/vuejs/vue/compare/v2.5.20...v2.5.21

)

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