rubocop: lint for Ruby 2.7

Author: bastelfreak

.rubocop.yml

@@ -1,4 +1,6 @@
 ---
+inherit_from: .rubocop_todo.yml
+
 require:
   - rubocop-performance
   - rubocop-rake
@@ -9,7 +11,7 @@ AllCops:
   DisplayCopNames: true
   ExtraDetails: true
   DisplayStyleGuide: true
-  TargetRubyVersion: '2.6'
+  TargetRubyVersion: '2.7'
   Exclude:
   - vendor/**/*
   - .vendor/**/*