Add a test for Leiningen

Add a simple test for Leiningen

```
$ lein --version
brew tetLeiningen 2.3.4 on Java 1.7.0_10 Java HotSpot(TM) 64-Bit Server VM
$ brew test leiningen
Testing leiningen
==> /usr/local/Cellar/leiningen/2.3.4/bin/lein --version
$ 
```

This follows the wget formula example.