Use Leiningen's Exit Semantics

lein-midje is currently using `System/exit` after it's done (with status 255 in case of error and 0 otherwise). This breaks higher-order tasks (like `do`) which rely on `leiningen.core.main/exit`'s behaviour to surpress termination of the process if desired. For example:

```
$ lein do version, midje
Leiningen 2.2.0 on Java 1.7.0_25 Java HotSpot(TM) 64-Bit Server VM
All checks (9) succeeded.

$ lein do midje, version
All checks (9) succeeded.
```

This commit makes lein-midje only call `exit` in case of failure:

```
$ lein do midje, version
All checks (9) succeeded.
Leiningen 2.2.0 on Java 1.7.0_25 Java HotSpot(TM) 64-Bit Server VM
```