RuboCop: Bump to Ruby 2.6

Author: eregon

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.6
   DisabledByDefault: true
   DisplayCopNames: true
   ExtraDetails: true
@@ -175,10 +175,6 @@ Lint/InheritException:
   Exclude:
   - src/main/ruby/core/exception.rb
 
-Lint/InvalidCharacterLiteral:
-  Description: Checks for invalid character literals with a non-escaped whitespace character.
-  Enabled: true
-
 Lint/LiteralInCondition:
   Description: Checks of literals used in conditions.
   Details: >-