Fix for partitioning a single node

Previous code was creating a grudge like:
INFO  jepsen.nemesis - {:n1 #{#<clojure.lang.PersistentList$1@1fbc22da>}, :n2 #{#<clojure.lang.PersistentList$1@1fbc22da>}, :n4 #{#<clojure.lang.PersistentList$1@1fbc22da>}, :n3 #{#<clojure.lang.PersistentList$1@1fbc22da>}, #<clojure.lang.PersistentList$1@1fbc22da> #{:n3 :n4 :n2 :n1}, :n5 #{#<clojure.lang.PersistentList$1@1fbc22da>}}

Perhaps it's an issue with clojure versions? I'm using 1.5.1 here. Ignore if thats the case.