Don't fail when 'did_you_mean' is not available.

Although 'did_you_mean' is always available in upstream Ruby, it might
not be available when speed, memory, disk or network bandwidth is a
concern. This is typically the case for production environment, where
'did_you_mean' is of limited usage.

Also it might not be available optionally when Ruby is delivered via
packaging systems. E.g. Linux distributions.

gem_prelude.rb

@@ -1,2 +1,6 @@
 require 'rubygems.rb' if defined?(Gem)
-require 'did_you_mean' if defined?(DidYouMean)
+
+begin
+  require 'did_you_mean'
+rescue LoadError
+end if defined?(DidYouMean)