DSL: add `accessibility_access` stanza

* accepts a Boolean value
* replaces the never-documented `postflight` methods `enable_accessibility_access` /
 `disable_accessibility_access` (`remove_accessibility_access`)
* declarative: unlike the `postflight` approach, does not have to be
  invoked in two places
* de-document obsolete form `caveats` / `assistive_devices`
* bump Travis seed to avoid false test failures