Use @stable for running tests

@latest fails with an error about not finding the timeline package

Author: Mark Visser

.yamato/promotion.yml

@@ -23,7 +23,7 @@ promotion_test_{{ platform.name }}_{{ editor.version }}:
   variables:
     UPMCI_PROMOTION: 1
   commands:
-    - npm install upm-ci-utils@latest -g --registry https://api.bintray.com/npm/unity/unity-npm
+    - npm install upm-ci-utils@stable -g --registry https://api.bintray.com/npm/unity/unity-npm
     - upm-ci package test --unity-version {{ editor.version }} --package-path com.unity.formats.fbx
   artifacts:
     logs:
@@ -79,4 +79,4 @@ promote:
 {% for platform in test_platforms %}
     - .yamato/promotion.yml#promotion_test_{{ platform.name }}_{{ editor.version }}
 {% endfor %}
-{% endfor %}
+{% endfor %}