Promote methods to public on relay's array conn

I recently extracted a gem called
[graphql-page_cursors](https://github.com/artsy/graphql-page_cursors)
that extracts Artsy's graphql pagination approach but it relies on all
connections exposing first and last methods. Unless I'm missing
something, I believe in the case of Relay's Array connection these
methods have been marked private by mistake.

lib/graphql/relay/array_connection.rb

@@ -31,8 +31,6 @@ def has_previous_page
         end
       end
 
-      private
-
       def first
         @first ||= begin
           capped = limit_pagination_argument(arguments[:first], max_page_size)
@@ -47,6 +45,8 @@ def last
         @last ||= limit_pagination_argument(arguments[:last], max_page_size)
       end
 
+      private
+
       # apply first / last limit results
       def paged_nodes
         @paged_nodes ||= begin