Mocha exits with incorrect code when tests fail

Mocha exits with exit code 0 when tests fail.  This means it cannot be used with Travis-CI

http://about.travis-ci.org/docs/user/build-configuration/

```
The standard Unix exit code of “0” means the build passed; everything else is treated as failure.
```

Output

```
✖ 1 of 5 tests failed:


 % echo $?                                                              
 0
```
