Update to use symlinks if possible.

Actually `mkdirp.sync` calls `path.resolve` [here](https://github.com/substack/node-mkdirp/blob/master/index.js#L67), tries `fs.mkdirSync` [here](https://github.com/substack/node-mkdirp/blob/master/index.js#L70), and THEN calls `fs.statSync` [here](https://github.com/substack/node-mkdirp/blob/master/index.js#L86) in the case when the directory already exists. 

Unless I am missing something I think this guard will actually be faster.