requirements-optionals: use psycopg2-binary (#5969)

Ref: #5967 (comment)

Author: blueyed

requirements/requirements-optionals.txt

@@ -1,6 +1,6 @@
 # Optional packages which may be used with REST framework.
 pytz==2017.2
-psycopg2==2.7.4
+psycopg2-binary==2.7.4
 markdown==2.6.4
 django-guardian==1.4.9
 django-filter==1.1.0