Test against supported versions of ActiveSupport

[blimmer]

We currently are compatible with ActiveSupport 4 and 5.

atomic_cache/atomic_cache.gemspec

Lines 37 to 38 in 9bff227

	# Dependencies
	spec.add_dependency 'activesupport', '>= 4.2', '< 6'

We should explicitly run tests against latest 4.x and latest 5.x to ensure compatibility.

See https://github.com/Ibotta/atomic_cache/pull/3/files#r168928757 for the original comment from @onyxraven .