Run tests on Ruby 2.0 and remove deprecation warning

I've removed two warnings related to the deprecation of `Enumerator.new` in favor of `Object#to_enum` in Ruby 2.0, and enabled this version in `travis.yml`.

All tests are green running `script/test_all` for me in Ruby versions 1.8.7-p371, 1.9.3-p392, and 2.0.0-p0. Let me know if it's all good.