RHEL 6: ruby: Fix fallback to rake/rdoctask for old rake.

ruby/Rakefile.in

@@ -20,9 +20,10 @@ require 'rake/clean'
 require 'rake/testtask'
 
 # Used to be rake/rdoctask.  Now it's rdoc/task.
+# http://stackoverflow.com/questions/2460891/how-do-i-rescue-from-a-require-no-such-file-to-load-in-ruby
 begin
   require 'rdoc/task'
-rescue
+rescue LoadError
   require 'rake/rdoctask'
 end