remove additional f18 references

f18 has been EOL for 6 weeks now, time to purge it from devstack

Change-Id: I5aac2c63b2f4cd8b01ae685b1acf4c188637558b
diff --git a/files/rpms/horizon b/files/rpms/horizon
index 59503cc..2dd24e0 100644
--- a/files/rpms/horizon
+++ b/files/rpms/horizon
@@ -16,8 +16,8 @@
 python-migrate
 python-mox
 python-nose
-python-paste        #dist:f18,f19,f20
-python-paste-deploy #dist:f18,f19,f20
+python-paste        #dist:f19,f20
+python-paste-deploy #dist:f19,f20
 python-routes
 python-sphinx
 python-sqlalchemy