update gce image

Author: evandbrown

packer.json

@@ -10,7 +10,7 @@
       "type": "googlecompute",
       "account_file": "{{user `account_file`}}",
       "project_id": "{{user `project_id`}}",
-      "source_image": "ubuntu-14-04",
+      "source_image": "ubuntu-1404-trusty-v20150909a",
       "zone": "us-central1-a",
       "image_name": "redmine-{{timestamp}}-{{user `git_branch`}}-{{user `git_commit`}}"
     },