bumps clojure to 1.5.1

clojure 1.5.0 had a memory leak bug. The fact that no one has reported
any issues with riemann suggests that riemann is not effected by the
bug, but I still feel better with the patched version of 1.5.1. :)
Merge pull request #237 from bmabey/master

bumps to clojure 1.5.1 for memory leak patch