upgrade to Ruby 2.0

the code still works as-is; no compatibility changes are needed
remove encoding comments thanks to Ruby 2.0

Now the project *requires* Ruby 2.0 – UTF-8 characters in my files could potentially cause bugs in other Ruby versions. The README now reflects this.
assert Ruby 2.0.0 in Gemfile, too

This change checks for Ruby 2.0.0 even if rbenv isn’t installed, as long as you use `bundle exec`. That’s an extra safety measure.

Also, Heroku uses that line to automatically choose the correct version of Ruby: https://devcenter.heroku.com/articles/ruby-versions . This program doesn’t use Heroku, but I can imagine that other software that others might use with this program might detect the version of Ruby in the same way.