chore(requirements): update idna to 2.4

See https://github.com/kjd/idna/blob/master/HISTORY.rst#24-2017-03-01

Author: mboersma

rootfs/requirements.txt

@@ -7,7 +7,7 @@ django-guardian==1.4.6
 djangorestframework==3.5.4
 docker-py==1.10.6
 gunicorn==19.6.0
-idna==2.2
+idna==2.4
 jmespath==0.9.1
 jsonfield==1.0.3
 jsonschema==2.6.0