Update _package.json

Increase required `grunt-sass` version up to [`0.13.0`](https://github.com/sindresorhus/grunt-sass/releases/tag/v0.13.0) so `yeogurt` will always support `.scss` indented (tabbed) syntax with minimum [`0.9.0`](https://github.com/sass/node-sass/releases/tag/v0.9.0) version of `node-sass` that requiring [`2.0`](https://github.com/sass/libsass/releases/tag/v2.0) version of `libsass`.

It's very important feature for me.
Merge pull request #36 from Grawl/patch-1

Update _package.json to update grunt-sass to v0.13.0