Fix bug in accessing a property using a variable

Wrapped the variable in curly braces to force it to evaluate before
trying to access the property.

app/Helpers/Links.php

@@ -39,10 +39,10 @@ class Links
                         || strpos($query_param, 'since_id=') === 0)
                     {
                         # Construct new links with the correct offset.
-                        $this->$link['rel'] = route($route) . '?' . $query_param;
+                        $this->{$link['rel']} = route($route) . '?' . $query_param;
                     }
                 }
             }
         }
     }
-}
+}