Added Elasticsearch 5 backend

wagtail/wagtailsearch/backends/elasticsearch5.py

@@ -0,0 +1,10 @@
+from __future__ import absolute_import, unicode_literals
+
+from .elasticsearch2 import Elasticsearch2SearchBackend
+
+
+class Elasticsearch5SearchBackend(Elasticsearch2SearchBackend):
+    pass
+
+
+SearchBackend = Elasticsearch5SearchBackend