Bump vuepress from 1.5.2 to 1.5.4 (#195)

Bumps [vuepress](https://github.com/vuejs/vuepress/tree/HEAD/packages/vuepress) from 1.5.2 to 1.5.4.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/v1.5.4/packages/vuepress)

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