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>
(TK-45) Add string support to `:scheme:` option

Allow the `:scheme` option for a proxy to take a string
as well as a keyword.
Merge pull request #56 from kbarber/maint/master/clojars-version-badge

Add a clojars version badge to the README.md
Merge pull request #61 from fpringvaldsen/TK-45-fix-proxy-schema

(TK-45) Add string support to `:scheme:` option