Don't fail when 'RubyGems' are not available.

Although 'RubyGems' are always available in upstream Ruby, it might not
be available optionally when Ruby is delivered via packaging systems.
E.g. Linux distributions.

gem_prelude.rb

@@ -1,4 +1,7 @@
-require 'rubygems.rb' if defined?(Gem)
+begin
+  require 'rubygems.rb'
+rescue LoadError
+end if defined?(Gem)
 
 begin
   require 'did_you_mean'