Disable pull of updated Shippable docker image.

This should improve reliability of our jobs on Shippable.

The Shippable AMIs should already have the latest docker image,
per Shippable support, so pull doesn't provide any benefit for us.

(cherry picked from commit 25caa67d41)

shippable.yml

@@ -62,6 +62,9 @@ matrix:
       python: 2.7
 build:
   pre_ci_boot:
+    image_name: drydock/u14pyt
+    image_tag: prod
+    pull: false
     options: "--privileged=false --net=bridge"
   ci:
     - test/utils/shippable/ci.sh