Bugfix: ActionView::Template::Error (no implicit conversion of Time into String)

Fix this error (`last_use` is stored as `Time` instance, actually)

```
2014-08-14T18:02:09.800221+00:00 app[web.1]: Started GET "/" for 110.134.150.31 at 2014-08-14 18:02:09 +0000
2014-08-14T18:02:09.828408+00:00 app[web.1]: Cache read: apps
2014-08-14T18:02:10.284036+00:00 app[web.1]:   Rendered apps/index.html.erb (453.7ms)
2014-08-14T18:02:10.331505+00:00 app[web.1]: 
2014-08-14T18:02:10.331535+00:00 app[web.1]:     31:                   <td><%= index + 1 %>.</td>
2014-08-14T18:02:10.331538+00:00 app[web.1]:     32:                   <td><%= link_to_if staging_url.match(/\Ahttps?:\/\//), staging_url, staging_url, {target: "_blank"} %></td>
2014-08-14T18:02:10.331539+00:00 app[web.1]:     33:                   <td><%= server['branch_name'] %></td>
2014-08-14T18:02:10.331541+00:00 app[web.1]:     34:                   <td><time datetime="<%= server['last_use'] %>"><%= time_ago_in_words(Time.parse(server['last_use'])) + " ago" %></time></td>
2014-08-14T18:02:10.331512+00:00 app[web.1]: ActionView::Template::Error (no implicit conversion of Time into String):
2014-08-14T18:02:10.331546+00:00 app[web.1]:     37:             </table>
2014-08-14T18:02:10.331544+00:00 app[web.1]:     36:               <% end %>
2014-08-14T18:02:10.331552+00:00 app[web.1]:   app/views/apps/index.html.erb:28:in `block in _app_views_apps_index_html_erb___3204376534231082978_70138278195300'
2014-08-14T18:02:10.331549+00:00 app[web.1]:   app/views/apps/index.html.erb:28:in `each'
2014-08-14T18:02:10.331547+00:00 app[web.1]:   app/views/apps/index.html.erb:34:in `block (2 levels) in _app_views_apps_index_html_erb___3204376534231082978_70138278195300'
2014-08-14T18:02:10.331557+00:00 app[web.1]: 
2014-08-14T18:02:10.331555+00:00 app[web.1]:   app/views/apps/index.html.erb:12:in `_app_views_apps_index_html_erb___3204376534231082978_70138278195300'
2014-08-14T18:02:10.331543+00:00 app[web.1]:     35:                 </tr>
2014-08-14T18:02:10.299011+00:00 app[web.1]: Completed 500 Internal Server Error in 487ms
2014-08-14T18:02:10.331558+00:00 app[web.1]: 
2014-08-14T18:02:10.331553+00:00 app[web.1]:   app/views/apps/index.html.erb:12:in `each'
2014-08-14T18:02:10.331550+00:00 app[web.1]:   app/views/apps/index.html.erb:28:in `with_index'
```