ring: alias #count as #size

This is for backwards compatibility. Pry Rails uses `#size`:
77f770f376/lib/pry-rails/prompt.rb (L23)

lib/pry/ring.rb

@@ -24,6 +24,7 @@ class Pry
     # @return [Integer] how many objects were added during the lifetime of the
     #   ring
     attr_reader :count
+    alias size count
 
     # @param [Integer] max_size Maximum buffer size. The buffer will start
     #   overwriting elements once its reaches its maximum capacity