Fix simple RuboCop offenses and update TODO file (#1062)

* Fix Layout/EmptyLineAfterMagicComment offense

* Fix Layout/ExtraSpacing offense

* Fix Layout/ClosingParenthesisIndentation offenses

* Fix Style/MutableConstant offense

* Fix Style/UnneededInterpolation offenses

* Fix Style/RedundantParentheses offenses

* Update TODO config for RuboCop

* Add executable bit to test/test_helper.rb

ref: https://travis-ci.org/Shopify/liquid/jobs/488169512#L578

lib/liquid/i18n.rb

@@ -26,7 +26,7 @@ module Liquid
     def interpolate(name, vars)
       name.gsub(/%\{(\w+)\}/) do
         # raise TranslationError, "Undefined key #{$1} for interpolation in translation #{name}"  unless vars[$1.to_sym]
-        "#{vars[$1.to_sym]}"
+        (vars[$1.to_sym]).to_s
       end
     end