Fix person spec, use `first(15)` instead of `slice(0..14)`

spec/lib/stream/person_spec.rb

@@ -32,7 +32,7 @@ describe Stream::Person do
     end
 
     posts = posts.reverse.slice(0..14)
-    fetched_posts = fetched_posts.slice(0..14)
+    fetched_posts = fetched_posts.first(15)
 
     expect(fetched_posts).to eq(posts)
   end