Change target Ruby version for Rubocop to 2.1.

We have to use the lowest common denominator to check the supported syntax
and in our case it is Ruby 2.1.
Please note that it will not help with unsupported syntax in HAML files
because they are not checked by Rubocop.

.rubocop.yml

@@ -5,7 +5,7 @@ require:
 inherit_from: .rubocop_todo.yml
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.1
   # Cop names are not d§splayed in offense messages by default. Change behavior
   # by overriding DisplayCopNames, or by giving the -D/--display-cop-names
   # option.