Hangman game in Ruby
It looks good and is readable.

A few notes :


current_state = "#{@display_content}" could be written current_state = @display_content.clone
words[rand(words.length)].strip could be words.sample.strip
select { |word| word.length.between?(5, 12) } will select words with a length between 4 and 11 due to the newline.
It would be nice to have some more feedback:


when the game has been successfully saved
when the game has been loaded, what the drawing looks like
when the last letter has been guessed, what the complete word was. Here's an example output:





_u_ar_
4 turns left
Enter a letter or attempt the full word: s
  ______
 |      |
(oo)    |
/||\    |
        |

su_ars
4 turns left
Enter a letter or attempt the full word: g
  ______
 |      |
(oo)    |
/||\    |
        |

You found the correct word!



You could replace the whole case;when statement with an array of strings.


Keep up the good work!