Fixed docco dependency name.

Before (i.e. on `v0.2.0`) running `npm ls` on a project using `grunt-docco2` yielded

```bash
...
npm ERR! missing: docco-dev@git+https://git@github.com/jashkenas/docco.git#d6a7f7dc941232bcbbb4b7dd0feb70ca735f9fab, required by grunt-docco2@0.2.0
npm ERR! extraneous: docco@0.6.2 /my_module/node_modules/grunt-docco2/node_modules/docco
```

which e.g. caused `npm-shrinkwrap` to fail.

Also, I realized there is a fresh `docco` release (newer than the commit referenced before), so I went back to requiring an actual version.