Drop support for Drupal 8 in 2.1.x.

config/schema/rest_views.image.schema.yml

-field.formatter.settings.rest_views_image:
-  type: field.formatter.settings.image
-  mapping:
-    export_alt:
-      type: boolean
-      label: 'Export the alt attribute'
-    export_title:
-      type: boolean
-      label: 'Export the title attribute'