Update leiningen to 2.3.0.

Unfortunately, leiningen will now pull in some dependencies via maven (via http) on `lein version' so the test at the end of builder.sh failed. This is okay because leiningen is used only as a interactive tool and no other package in Nixpkgs depends on it.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Merge pull request #812 from the-kenny/leiningen-2.3.0

Update leiningen to 2.3.0.