Unhandled CaseClauseError from bug in Posion

[dbernheisel]

When Shopify.Response is trying to parse the body of the response, I'm seeing a CaseClauseError exception occur sometimes from this line. It's because of a bug in Poison (I'm using Poison version 3.1.0). It's not a documented signature, so I understand it may not be this shopify lib's responsibility to fix it, however we could prevent it by modifying the mix.exs requirement for poison to either avoid this bugged version, or add a clause to capture it.

Poison 4.x returns to a 2-item error tuple so it should be compatible again.