node-sass: workaround missing vendor folder

[n-riesco]

After a fresh install:

git clone https://github.com/plotly/react-chart-editor.git
cd react-chart-editor
npm install

running npm run make:lib:css fails because of issue sass/node-sass#1579.

This PR adds a postinstall script to rebuild node-sass and fix this issue.

[nicolaskruchten]

Just for context: when would you want/need to run npm run make:lib:css as a consumer of this library? Or is this just for clean installs of the library itself...?