title: Decorators problem: experimental syntax 'decorators-legacy' isn't currently enabled
body: <p>i'm a very gatsbyjs beginner.
  I've a very simple site generated from &quot;hello word&quot; starter.
  I added gatsby-plugin-sass and bootstrap.
  In my index.js when I wrote
  @import &quot;~bootstrap/scss/bootstrap.scss&quot;;</p>
  <p>I obtain
  /src/pages/index.js: Support for the experimental syntax 'decorators-legacy' isn't currently enabled (4:1)</p>
  <p>what's the matter?</p>