Switch rubocop to one of the modern ruby versions

Signed-off-by: Artem Sidorenko <[email protected]>

Author: artem-sidorenko

.rubocop.yml

@@ -4,7 +4,7 @@ AllCops:
   Exclude:
   - vendor/**/*
   - test/**/*
-  TargetRubyVersion: 2.1 # we need this because of chef 12.5.1 support
+  TargetRubyVersion: 2.4
 Metrics/AbcSize:
   Max: 29
 Metrics/CyclomaticComplexity:
@@ -35,4 +35,4 @@ Metrics/BlockLength:
   Exclude:
     - 'spec/**/*'
 Style/FrozenStringLiteralComment:
-  Enabled: false
+  Enabled: false