Cache-busting for loading elasticsearch dashboards

When using Internet Explorer, if you save changes to an elasticsearch dashboard and then reload the page or switch between dashboards, the "old" prior version of the dashboard comes back and the changes appear to have been lost.

I believe this is an IE caching issue which was introduced at [elasticsearch/kibana/aa4b3ef0:js/services.js#757](https://github.com/elasticsearch/kibana/commit/aa4b3ef0472947623ff69c1d963fc5dd434e62fb#diff-f536afa50a9dd6df0ca45f748f32326eR757) when it switched from using `POST /kibana-int/dashboard/_search` to a `GET /kibana-int/dashboard/{name}`.

I added a cache-busting token in the same manner as [`file_load`](https://github.com/elasticsearch/kibana/blob/41046796c80ac22c3b969169a317ffdb3ca7581d/src/app/services/dashboard.js#L277) to help work around IE's over-caching of `GET` ajax requests.