Bump up allowed Poison version

[mdoza]

Some Phoenix libraries like Addict are not yet ready for Phoenix 1.2 due to dependency issues. Mailgun is one, since it is not allowing Poison beyond 2.0. When creating new Phoenix 1.2 applications, Poison is being locked at 2.2.0.

[jastax]

Yep, I'm at the same point. A quick change would be awesome :)

[derekbrown]

@chrismccord Pinging for review. Is this repo abandoned?

[hankditton]

Might as well make this also OK with poison 3.0 and above at this point (though it doesn't seem likely that this will be merged).