Update the location of the Elastic License

Not a 404 since it points to a specific tag, but there was a license update and the new link points to what you are currently getting.

elasticsearch/license.md

@@ -1 +1 @@
-View [license information](https://github.com/elastic/elasticsearch/blob/6.4/licenses/ELASTIC-LICENSE.txt) for the software contained in this image.
+View [license information](https://github.com/elastic/elasticsearch/blob/master/licenses/ELASTIC-LICENSE-2.0.txt) for the software contained in this image.