Workaround for rb_block_given_p() return value change in Ruby 1.9 series...

Fix for issue mentioned at trema/trema@be7f0725894a120c3c486bed3583ed4e3ee1980f

See: 
 http://transitive.info/2011/02/18/note-rb_block_given_p/

cruise.rb script passed on Ubuntu Server 12.04 LTS using following Ruby version installed using RVM.
 * ruby-1.8.7-p374 [ x86_64 ]
 * ruby-1.9.3-p448 [ x86_64 ]
 * ruby-2.0.0-p247 [ x86_64 ]