use json.parse only when returning data is string

Hi, I found that when I run the test, it's throw an error "SyntaxError: Unexpected token o".

```javascript
console.log('data'):
{ kind: 'pagespeedonline#result',
  id: 'https://www.google.co.th/?gws_rd=cr,ssl&ei=SKU7VP3hEMrc8AWu-YHQCw',
  responseCode: 200,
  title: 'Google',
 ...
 ...
  version: { major: 1, minor: 15 } }

SyntaxError: Unexpected token o
    at Object.parse (native)
    at /home/sakp/testpsi/node_modules/psi/index.js:27:25
```