Update gemspec

This test fails https://github.com/ruby/ruby/runs/6216873955?check_suite_focus=true

The sync tool changes the path structure of the gem so the version file is no longer in the same relative location.

lib/dead_end/dead_end.gemspec

@@ -1,6 +1,10 @@
 # frozen_string_literal: true
 
-require_relative "lib/dead_end/version"
+begin
+  require_relative "lib/dead_end/version"
+rescue LoadError # Fallback to load version file in ruby core repository
+  require_relative "version"
+end
 
 Gem::Specification.new do |spec|
   spec.name = "dead_end"