updates for ruby 2.0

* I changed the deprecated `Enumerate.new`
* `line.encode!(…)` as 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 because converting the encoding from UTF8 to UTF8 apparently doesn't do anything ([#7967](http://bugs.ruby-lang.org/issues/7967#note-1)), hence I changed the source to binary.