Use 6.1.0 version of elasticsearch-dsl

Till we support `elasticsearch-dsl>6.1.0`, we should keep using `elasticsearch-dsl<6.1.0`

Author: safwanrahman

setup.py

@@ -42,7 +42,7 @@
     ],
     include_package_data=True,
     install_requires=[
-        'elasticsearch-dsl>=2.1.0,<7.0.0',
+        'elasticsearch-dsl>=2.1.0,<6.1.0',
     ],
     license="Apache Software License 2.0",
     zip_safe=False,