Fix cargo.toml

The latest commit to master breaks cargo (cargo 0.0.1-pre-nightly (bddcb49 2014-08-15 04:07:00 +0000)). This reverts that change.

The error message was:

```
Cargo.toml is not a valid manifest

expected a value of type `array`, but found a value of type `table` for the key `lib`
```