title: Singularity multiple gutters being ignored
body: <p>I'm using Singularity.gs 1.5.1 to produce a grid. When I declare additional gutter dimensions they are ignored. This is what I have:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>@include add-grid(12);
  @include add-gutter(20px);
  @include add-gutter(30px at 640px);
  @include add-gutter(40px at 1024px);
  @include add-gutter-style('split' 'fixed');
  @include sgs-change('output', 'float');
  
  .column-1 {
    @include grid-span(1, 1);
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>This is producing 20px grids at all screen sizes. According to the docs this should produce:</p>
  
  <p>20px gutters at 0 - 640px <br>
  30px gutters at 640px - 1024px <br> 
  40px gutters at 1024px and above <br></p>
  
  <p>Why is this not working?</p>