Update CI rubies
Add default Rake task, remove unneccessary bundler tasks
Don't freeze value or reason

This has been causing me lots of pain, and it only freezes
shallowly anyway. The problematic case is when the return
of a callback is passed to the next promise. Unless you
make `nil` the last expression of that callback, one of
your precious objects will get frozen, and you'll only
notice when you eventually try to modify it.

Apart from that, immutability is a nice in the Ruby world,
and given that I want promise.rb to play nice with others,
I'm just gonna disregard what the JS-centered A+ spec says.