Fixes for Clojure 1.6 compatibility

The new version of Clojure made some changes to .hashCode for various
data structures, so places where we were relying on (undefined)
insertion-order traversal of various data structures stopped working.
(PDB-512) Update to Clojure 1.6.0-RC1
Merge pull request #887 from grimradical/clj-1.6-compat

Fixes for Clojure 1.6 compatibility