Add key to pageLinks within SiteIndex

* Key is required when creating a list of elements
* Adding key removes react warning
* Used title because it should be unique
* Closes #25

pages/index.jsx

@@ -23,7 +23,7 @@ class SiteIndex extends React.Component {
                 const category = access(page, 'data.category')
 
                 pageLinks.push(
-                    <div className='blog-post'>
+                    <div className='blog-post' key={ title }>
                       <time dateTime={ moment(datePublished).format('MMMM D, YYYY') }>
                         { moment(datePublished).format('MMMM YYYY') }
                       </time>