Fix firefox assertions.

They were not using the proper attribute.

playbooks/roles/jenkins_worker/tasks/test_platform_worker.yml

@@ -23,7 +23,7 @@
   register: firefox_version
 - assert:
     that:
-      - "'28.0' in firefox_version"
+      - "'28.0' in firefox_version.stdout"
 
 # The role is run with a github oauth token passed in
 # as github_oauth_token var value.

playbooks/roles/jenkins_worker/tasks/test_sitespeed_worker.yml

@@ -12,4 +12,4 @@
   register: firefox_version
 - assert:
     that:
-      - "'40.0' not in firefox_version"
+      - "'28.0' not in firefox_version.stdout"