fix underscore.string version 2.2.0rc to 2.2.0-rc

Hi, I can't install gruntjs.

```
$ npm install grunt
```

then error

```
npm ERR! Error: No compatible version found: underscore.string@'>=2.2.0rc <2.3.0-'
npm ERR! Valid install targets:
npm ERR! ["0.9.2","1.0.0","1.1.3","1.1.4","1.1.5","1.1.6","2.0.0","2.1.0","2.1.1","2.3.0","2.3.1","2.2.0-rc"]
```

so I fixed ```package.json``` dependencies version.

thanks.