Bump psycopg2 from 2.7.7 to 2.8

Bumps [psycopg2](https://github.com/psycopg/psycopg2) from 2.7.7 to 2.8.
- [Release notes](https://github.com/psycopg/psycopg2/releases)
- [Changelog](https://github.com/psycopg/psycopg2/blob/master/NEWS)
- [Commits](https://github.com/psycopg/psycopg2/commits)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements/base.txt

@@ -12,7 +12,7 @@ django-toolbelt==0.0.1
 gunicorn==19.9.0
 gviz_api==1.9.0
 py-bcrypt==0.4
-psycopg2==2.7.7
+psycopg2==2.8
 pytz==2018.9
 requests==2.21.0
 static==1.1.1