chore: add ruby 2.7 compatibility

README.md

@@ -92,7 +92,7 @@ Choose version:
 
 | paper_trail | ruby     | activerecord  |
 |-------------|----------|---------------|
-| unreleased  | >= 3.0.0 | >= 6.1, < 7.2 |
+| unreleased  | >= 2.7.0 | >= 6.1, < 7.2 |
 | 15          | >= 3.0.0 | >= 6.1, < 7.2 |
 | 14          | >= 2.7.0 | >= 6.0, < 7.1 |
 | 13          | >= 2.6.0 | >= 5.2, < 7.1 |