Update builder revisions to use new format

https://codereview.chromium.org/475943002 updated the
backend builder info to include status and last update
time instead of just revisions. Updating ct-builder-revisions
and its test to use the new format.

TBR=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/480213002

git-svn-id: svn://svn.chromium.org/blink/trunk@180491 bbb929c8-8fbe-4397-9dbb-9b2b20218538