title: "IndexError: Too many levels" when running Featuretools dfs after upgrade
body: <p>Featuretools' <code>dfs()</code> method fails to run on my entity set after upgrading from v0.1.21 to v0.2.x and v0.3.0.</p>
  
  <p>The error is raised when the Pandas backend tries to calculate the aggregate features <code>_calculate_agg_features()</code>. In particular:</p>
  
  <pre><code>--&gt; 442 to_merge.reset_index(1, drop=True, inplace=True)
  ...
  IndexError: Too many levels: Index has only 1 level, not 2
  </code></pre>
  
  <p>This is working fine in v0.1.x and the entity set hasn't changed after the upgrade. The entity set is composed of 7 entities and 6 relationships. Each entity (dataframe) is added via <code>entity_from_dataframe</code>.</p>