Bump the Rubocop `TargetRubyVersion` parameter now we're using 2.6.3

- This was set to 2.3 because Homebrew was running Ruby 2.3.7. Since
  https://github.com/Homebrew/brew/pull/6556, we're running 2.6.3. With
  this change, Rubocop will check against modern Ruby syntax.

Library/.rubocop_shared.yml

@@ -3,7 +3,7 @@
 require: ./Homebrew/rubocops.rb
 
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.6
   DisplayCopNames: false
 
 # Use `<<~` for heredocs.