v1.5.1

Added

• Coverage.new_gap_hint sets the advice the baseline-mismatch message
  gives for a new gap. The default (Coverage::DEFAULT_NEW_GAP_HINT) is
  unchanged; a host whose baseline only shrinks can drop "or add it to the
  baseline".

Fixed

• RBS for a subclass of a typed action now resolves on its own. Its result
  classes subclass the parent's (< ::ApiAuth::BaseAction::Success), which
  nothing declared when the parent is an abstract base that is never
  generated. RBS.generate now declares those parent classes, empty, up the
  chain of typed ancestors.
• RBS readers on Failure are nullable. An invalid_output failure
  carries the output that broke its schema, so a required field can be
  missing; the readers were typed non-nil.