Replaced {{author}} by {{primary_author}} in byline-single.hbs

refs https://github.com/TryGhost/gscan/issues/99

- single author usages will be deprecated in Ghost 2.0
- make Casper 100% compatible with Ghost 2.0

partials/byline-single.hbs

@@ -1,5 +1,5 @@
 {{!-- Everything inside the #author tags pulls data from the author --}}
-{{#author}}
+{{#primary_author}}
 
 <section class="author-card">
     {{#if profile_image}}
@@ -20,4 +20,4 @@
     <a class="author-card-button" href="{{url}}">Read More</a>
 </div>
 
-{{/author}}
+{{/primary_author}}