Commits on Sep 1, 2019

1. Update @vue/unit-mocha install instructions …

   Similar to [this comment](#4294 (comment)) addressing vue-jest install issues, the install instructions for `@vue/unit-mocha` have also changed and need to be updated too.
   
   #### Before change:
   
   ```bash
   vue add unit-mocha
   
   📦  Installing vue-cli-plugin-unit-mocha...
   
   npm ERR! code E404
   npm ERR! 404 Not Found - GET https://registry.npmjs.org/vue-cli-plugin-unit-mocha - Not found
   npm ERR! 404 
   ```
   
   #### After change
   
   ```bash
   vue add @vue/unit-mocha
   
   📦  Installing @vue/cli-plugin-unit-mocha...
   
   + @vue/cli-plugin-unit-mocha@3.11.0
   added 70 packages from 505 contributors and audited 29226 packages in 17.477s
   ```

   

   devjack committed Sep 1, 2019
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