Make elasticsearch-py a dev requirement

The Elasticsearch tests now depend on an environment variable being set so we can safely install elasticsearch-py without Wagtail automatically assuming Elasticsearch is installed and running the tests.

This makes developer setup slightly easier, but also opens up the possibility for running some of the ES tests that don't depend on a running ES instance.

docs/contributing/developing.rst

@@ -90,10 +90,8 @@ the tests against it.
 
 **Testing Elasticsearch**
 
-To test Elasticsearch, you need to have the ``elasticsearch`` package installed.
-
-Once installed, you can test Wagtail with Elasticsearch by passing the
-``--elasticsearch`` argument to ``runtests.py``::
+You can test Wagtail against Elasticsearch by passing the ``--elasticsearch``
+argument to ``runtests.py``::
 
     python runtests.py --elasticsearch
 

requirements-dev.txt

@@ -3,6 +3,7 @@ mock>=1.0.0
 python-dateutil>=2.2
 pytz>=2014.7
 Pillow>=2.7.0
+elasticsearch>=1.0.0
 
 # For coverage and PEP8 linting
 coverage>=3.7.0