Use &block instead of Proc.new

Using Proc.new to implicitly capture a passed block was removed from
Ruby 3.0.

Author: rossta

lib/serviceworker/router.rb

@@ -10,10 +10,10 @@ def self.default
 
     attr_reader :routes
 
-    def initialize
+    def initialize(&block)
       @routes = []
 
-      draw(&Proc.new) if block_given?
+      draw(&block)
     end
 
     def draw(&block)