contributing.md django fix. (#4581)

* installation for django added in requirements.txt
* added line to install django first in contributing.md

* added line to install django first in contributing.md and CONTRIBUTING.md

CONTRIBUTING.md

@@ -61,6 +61,7 @@ To run the tests, clone the repository, and then:
     # Setup the virtual environment
     virtualenv env
     source env/bin/activate
+    pip install django
     pip install -r requirements.txt
 
     # Run the tests

docs/topics/contributing.md

@@ -61,6 +61,7 @@ To run the tests, clone the repository, and then:
     # Setup the virtual environment
     virtualenv env
     source env/bin/activate
+    pip install django
     pip install -r requirements.txt
 
     # Run the tests