Use dev_dependencies in favour of dependencies for unittest

This way I can get rid of this message on running ``` pub get ```

```

--- Feb 24, 2014 6:03:41 PM Running pub get ... ---
Pub get failed, [1] Resolving dependencies.................
Incompatible version constraints on unittest:
- forcemvc depends on version >=0.9.0 <0.9.1
- lexer depends on version 0.10.0
- myapp depends on version any


```

Because when myapp depends on lexer, Pub will not include the unittest version 0.10.0 anymore.