Pin djangorestframework to latest version 3.5.1

Author: pyup-bot

requirements/base.txt

@@ -54,7 +54,7 @@ django-user-sessions==1.3.1
 nexmo==1.4.0
 django-geoposition==0.3.0
 django-ordered-model==1.3.0
-djangorestframework
+djangorestframework==3.5.1
 markdown==2.6.7
 django-filter==0.15.3
 docutils==0.12