Update psycopg2-binary to version 2.8.5

2.8.4 is the first version providing manylinux wheels for
Python 3.8 so it should make no difference if we update it
to the latest version 2.8.5.

requirements.txt

@@ -1,5 +1,5 @@
 django>=1.11,<1.12
 django-debug-toolbar==1.8
 gunicorn==19.5.0
-psycopg2-binary==2.7.3.2
+psycopg2-binary==2.8.5
 whitenoise==3.3.1