Update to version 1.2.
Removed obsolete JUnit dependency.
Changed to FlatSpec as opposed to using FunSpec.
Added `atmos.retries.TerminationPolicy.ImmediatelyTerminate`, a policy
that never retries.
Removed the previous backoff value from the list of items provided to a
`BackoffPolicy` when calculating a backoff and replaced it with the most
recently thrown exception.
Added `BackoffPolicy.Selected`, a policy that selects another backoff
policy based on the most recent exception.
Added `BackoffPolicy.Randomized`, a policy that randomizes the result of
another backoff policy.
Release version 1.2.