Update select2-bootstrap.css

Added Bootstrap Validation states for `warning`, `error`, `info`, and `success`

Example use:

    <div class="control-group warning">
      <label class="control-label" for="e1">Input with warning</label>
      <div class="controls">
        <select id="e1">
          <option value="AL">Alabama</option>
          ...
          <option value="WY">Wyoming</option>
        </select>
        <span class="help-inline">Something may have gone wrong</span>
      </div>
    </div>