Bump laravel-mix from 2.1.14 to 3.0.0 (#200)

Bumps [laravel-mix](https://github.com/JeffreyWay/laravel-mix) from 2.1.14 to 3.0.0.
<details>
<summary>Release notes</summary>

*Sourced from [laravel-mix's releases](https://github.com/JeffreyWay/laravel-mix/releases).*

> ## v3.0
> ### New
> 
> **Babel 7 support!** As this new version of Babel [includes major breaking changes](https://babeljs.io/blog/2018/08/27/7.0.0#major-breaking-changes), Mix, too, needs to bump from v2 to v3. We've taken care of most the changes, however, if your project pulls in extra Babel plugins that Mix does not provide out of the box, you'll need to update locally as well. 
> 
> 1. The naming convention for official Babel plugins has changed. They are now scoped under the [**babel**](https://github.com/babel) namespace. As such, in your `package.json` file, change all occurrences of `"babel-plugin-[name]": "6.x"` to `"@babel/plugin-[name]": "7.x"`
> 2. If you've created a `.babelrc` file in your project, update all plugin name references. For example, update `"plugins": ["babel-plugin-transform-object-rest-spread"]` to `"plugins": ["@babel/plugin-proposal-object-rest-spread"]`
> 
> ### Fixes
> 
> - Updated the rootPath to the working directory of the node process. [#1719](https://github-redirect.dependabot.com/JeffreyWay/laravel-mix/issues/1719)
> - Fixed issues with minified css and purify. [#1634](https://github-redirect.dependabot.com/JeffreyWay/laravel-mix/issues/1634)
> 
> ### Onward
> 
> Next up, we'll be upgrading the Mix codebase to webpack 4. Stay tuned.
</details>
<details>
<summary>Commits</summary>

- See full diff in [compare view](https://github.com/JeffreyWay/laravel-mix/commits)
</details>
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