Add a clojars version badge to the README.md

This simplifies and automates the display of the latest version. Its a shame
its not text so it can't be cut & paste, but its better then nothing and
maintained by clojars, so no reliance on some startup SaaS service that may
disappear tomorrow.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #56 from kbarber/maint/master/clojars-version-badge

Add a clojars version badge to the README.md