Enabled frozen string literals (#1154)

* Enabled frozen string literals

* Update rubocop config

* Prefer string interpolation in simple cases

Co-Authored-By: Dylan Thacker-Smith <dylan.smith@shopify.com>

example/server/example_servlet.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module ProductsFilter
   def price(integer)
     format("$%.2d USD", integer / 100.0)

example/server/liquid_servlet.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class LiquidServlet < WEBrick::HTTPServlet::AbstractServlet
   def do_GET(req, res)
     handle(:get, req, res)

example/server/server.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'webrick'
 require 'rexml/document'