Use psycopg2 2.8 since < 2.8 prevents Python 3.8 support

https://github.com/psycopg/psycopg2/issues/854
diff --git a/requirements.server.txt b/requirements.server.txt
index 0e008b1..c1867e4 100644
--- a/requirements.server.txt
+++ b/requirements.server.txt
@@ -1,6 +1,6 @@
 # These requirements are what you would install on a production server.
 -r requirements.client.txt
-psycopg2==2.7
+psycopg2==2.8
 # Flask 0.11 does not work yet.
 gunicorn==18.0
 progressbar2