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/keystone b/files/rpms/keystone
index 99e8524..7182091 100644
--- a/files/rpms/keystone
+++ b/files/rpms/keystone
@@ -1,9 +1,9 @@
 python-greenlet
 libxslt-devel       # dist:f20
-python-lxml         #dist:f18,f19,f20
-python-paste        #dist:f18,f19,f20
-python-paste-deploy #dist:f18,f19,f20
-python-paste-script #dist:f18,f19,f20
+python-lxml         #dist:f19,f20
+python-paste        #dist:f19,f20
+python-paste-deploy #dist:f19,f20
+python-paste-script #dist:f19,f20
 python-routes
 python-sqlalchemy
 python-webob