updates for ruby 2.0

First things first: This is not yet ready to be merged, as it wont work!

I changed the deprecated `Enumerate.new`, which is fine by itself.

However the `line.encode!(…)` used in [encoding_aware_iterator.rb](https://github.com/square/cane/blob/97f1b30a5ff449eb8a1232d01f32ee07f5bf1fef/lib/cane/encoding_aware_iterator.rb#L18) doesn't work in ruby-2.0.0p0, for this I filed a bug on ruby's issue tracker ([#7967](http://bugs.ruby-lang.org/issues/7967)).