Loosen redis-rb dep

I notice you were specific about this dep in https://github.com/Sirupsen/progressrus/commit/0fabf7106165ba29cff52e2b3a76ea07767b9aef, but I want to loosen it to allow 3.1.0 so I can bump Shopify up to that version.

The tests passed for me on redis-rb 3.0.4, 3.0.6, and 3.1.0.

@Sirupsen