Widget engine install fails when the image module is not installed

When installing the module without the Image module installed, a fatal error is thrown about a missing "image" module.
This then leaves it in a partial state where it doesn't work properly, and the uninstall throws a database errors .

I don't have the errors to hand as I was rushing to get the site back up and running.

This patch adds the Image module to the dependancies.

diff -uar widget_engine.old/widget_engine.info.yml widget_engine/widget_engine.info.yml
--- widget_engine.old/widget_engine.info.yml    2018-04-26 19:14:56.160556281 +0100
+++ widget_engine/widget_engine.info.yml        2018-04-26 19:13:22.140556281 +0100
@@ -5,6 +5,7 @@
 package: Widget system
 dependencies:
   - inline_entity_form
+  - drupal:image
   - drupal:node
   - drupal:path
   - drupal:text