Remove warnings related to Enumerator.enw in ruby 2.0

I've removed two warnings related to the deprecation of `Enumerator.new` in favor of `Object#to_enum` in Ruby 2.0.

I took the opportunity to update some dependencies, skipping the rcov installation for Ruby versions other than 1.8. I've fixed a duplicated test name as well.

All tests are green for me in Ruby versions 1.8.7-p370, 1.9.3-p327, and 2.0.0-rc1. Let me know if it's all good.