misc/dashboard/app: trim old builds from the history

The dashboard is currently failing to store results of new builds for some keys, notable the go.codereview sub repository. This is causing the builders to mark the entire triggering commit as failed. With the help of David Symonds we think it is because the results value has breached the 1mb datastore limit on AppEngine.

R=dsymonds, adg
CC=golang-dev
https://codereview.appspot.com/6858094