Make gem compatible with Ruby >= 2.7 projects

[jmgarnier]

Fix:

ArgumentError:
       wrong number of arguments (given 3, expected 2; required keywords: file_path, line_index)
     # reverse_coverage-0.1.1/lib/reverse_coverage/main.rb:80:in `save_changes'
     # reverse_coverage-0.1.1/lib/reverse_coverage/main.rb:35:in `block (2 levels) in add'