Remove warnings related to Enumerator.enw in ruby 2.0

Warnings:

  ./rgl/lib/rgl/transitivity.rb:119:
      warning: Enumerator.new without a block is deprecated; use Object#to_enum
  ./rgl/lib/rgl/transitivity.rb:160:
      warning: Enumerator.new without a block is deprecated; use Object#to_enum

Changing to Object#to_enum fixes them, and all tests are green in:
1.8.7-p370, 1.9.3-p327, and 2.0.0-rc1.