Update active_utils to 3.0.0, change namespace.

@karlhungus @jnormore @arthurnn @wvanbergen this gets out of ActiveMerchant's bizness by implementing the namespace change, bumps the version, and of course depends on https://github.com/Shopify/active_utils/pull/36. It looks like a lot of changes, but that's just due to moving the files around; look at the commits individually for a better picture of what's changing.

Any feedback? Looking good to proceed?