Ensure RubyKoans works under Ruby 2.0

There are several if_ruby_version checks in edgecase.rb that don't check for 2.0, and the AssertionError definition needs to happen, so this is a tiny pull to address that.  Otherwise you just get the following:
```
ruby_koans/koans/edgecase.rb:399:in `rescue in meditate': uninitialized constant EdgeCase::Sensei::AssertionError (NameError)
```