chore(requirements): update idna to v2.6

See https://github.com/kjd/idna/blob/master/HISTORY.rst#26-2017-08-08

Author: mboersma

rootfs/requirements.txt

@@ -7,7 +7,7 @@ django-guardian==1.4.9
 djangorestframework==3.6.3
 docker-py==1.10.6
 gunicorn==19.7.1
-idna==2.5
+idna==2.6
 jmespath==0.9.3
 jsonfield==2.0.2
 jsonschema==2.6.0