Upgrade to nodeunit 0.8.0 to support node 0.10.5

Running many tests causes the following warnings.

```
(node) warning: Recursive process.nextTick detected. This will break in the next version of node. Please use setImmediate for recursive deferral.
```

This was fixed in nodeunit 0.8.0. Bumping the version number in the `package.json` seems to solve the issue.

See https://github.com/caolan/nodeunit/pull/195