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/swift b/files/rpms/swift
index 72253f7..bf29ea2 100644
--- a/files/rpms/swift
+++ b/files/rpms/swift
@@ -9,7 +9,7 @@
 python-greenlet
 python-netifaces
 python-nose
-python-paste-deploy # dist:f18,f19,f20,rhel7
+python-paste-deploy # dist:f19,f20,rhel7
 python-simplejson
 python-webob
 pyxattr