(FACT-3147) Set RuboCop target Ruby version to 2.5

Ruby 2.5 is the minimum recommended Ruby version for Facter. This
commit updates RuboCop's target Ruby version from 2.3 to 2.5.

Author: mhashizume

.rubocop.yml

@@ -2,7 +2,7 @@
 inherit_from: .rubocop_todo.yml
 
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.5
   Exclude:
     - acceptance/**/*
     - vendor/**/*
@@ -193,4 +193,4 @@ Style/HashTransformKeys:
   Enabled: false  # not implemented in ruby 2.3
 
 Style/HashTransformValues:
-  Enabled: false  # not implemented in ruby 2.3
+  Enabled: false  # not implemented in ruby 2.3