Update elasticsearch-dsl requirement

Due to some changes in elasticsearch-dsl we don't
support elasticsearch-dsl > 6.1 yet.

See issue #115

Author: sabricot

README.rst

@@ -38,7 +38,7 @@ Install Django Elasticsearch DSL::
     pip install django-elasticsearch-dsl
 
     # Elasticsearch 6.x
-    pip install 'elasticsearch-dsl>=6.0,<7.0'
+    pip install 'elasticsearch-dsl>=6.0,<6.2'
 
     # Elasticsearch 5.x
     pip install 'elasticsearch-dsl>=5.0,<6.0'

requirements.txt

@@ -1,2 +1,2 @@
 django>=1.9.6
-elasticsearch-dsl>=2.1.0,<7.0.0
+elasticsearch-dsl>=2.1.0,<6.2.0

requirements_dev.txt

@@ -1,5 +1,5 @@
 bumpversion==0.5.3
-wheel==0.29.0
-django>=2.0,<2.1
-elasticsearch-dsl>=2.1.0,<7.0.0
+wheel==0.32.2
+django>=2.0,<2.2
+elasticsearch-dsl>=2.1.0,<6.2.0
 -e .