Issue #89. Added note that EU doesn't work with ES 0.19.9.

The problem is this one:

https://github.com/elasticsearch/elasticsearch/issues/2205

This was a bug in ElasticSearch 0.19.9 that was fixed in 0.19.10 so
I added a note to the docs stating ElasticUtils doesn't work with
ElasticSearch 0.19.9.

docs/installation.rst

@@ -4,6 +4,15 @@
 Installation
 ============
 
+.. Warning::
+
+   ElasticUtils doesn't work well with ElasticSearch 0.19.9. If you're
+   using 0.19.9, you should update to at least 0.19.10.
+
+   https://github.com/elasticsearch/elasticsearch/issues/2205
+     ElasticSearch bug with ``_all``.
+
+
 There are a few ways to install ElasticUtils.