Kernel delegate

Hello! This is a small change that fixes mp3info for Ruby 2.1.0. It seems that subclasses of `DelegateClass` no longer handle `Kernel` methods properly -- for some reason, `#puts` and `#loop` are no-ops. Explicitly including `Kernel` corrects this and shouldn't do any harm to older Rubies (although I'll wait for Travis to verify that).

I took the liberty of including Ruby 2.1.0 in the Travis build while I was at it.