Don't reset Warning setup globally

What does this MR do and why?

Use Warning.clear(&block) to only reset the state within the spec examples.

Previously, any setup of Warning has been cleared via Warning.clear after this very spec has been run. Note that currently we don't have any warning processing defined but this will change soonish.

This MR also upgrades the gem warning to 1.3.0 which introduced Warning.clear {}.

Upgrade warning gem

• https://my.diffend.io/gems/warning/1.2.1/1.3.0

Changes:

* Allow Warning.clear to take a block, and restore current state after block (splattael) (#18, #20)
* Raise ArgumentError if Warning.process is passed non-String as first argument (splattael) (#17, #19)

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