buildbot_test_toolchains: get latest instead green vanilla build

So as to work around the problem that some builders' latest green
builds are too old.

BUG=chromium:714890
TEST=Tested _GetVanillaImageName locally.

Change-Id: I06571f3d55743fb56977d7ef56a51fcddfdda62e
Reviewed-on: https://chromium-review.googlesource.com/486086


Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Ting-Yuan Huang <laszio@chromium.org>
Tested-by: Ting-Yuan Huang <laszio@chromium.org>
Trybot-Ready: Ting-Yuan Huang <laszio@chromium.org>