Enabled frozen string literals (#1154)

* Enabled frozen string literals

* Update rubocop config

* Prefer string interpolation in simple cases

Co-Authored-By: Dylan Thacker-Smith <[email protected]>

lib/liquid/utils.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Liquid
   module Utils
     def self.slice_collection(collection, from, to)
@@ -69,7 +71,7 @@ module Liquid
       end
 
       case obj
-      when 'now'.freeze, 'today'.freeze
+      when 'now', 'today'
         Time.now
       when /\A\d+\z/, Integer
         Time.at(obj.to_i)