fixes #940

fix rubocop bugs
Merge pull request #942 from fshowalter/fix_940

fix useless_access_modifier bug
Minor changelog change
Enhance NegatedIf and NegatedWhile

They now handle properly `unless` and `until` constructs.
[Fix #925] Don't disable Syntax cop in rubocop-todo.yml

The Syntax cop is a fake cop and it can't be disabled in configuration.