Only do alerting on the last 50 commits.

I noticed that we get a flurry of alerts when we hit a tile boundary, so this is an attempt to smooth out that by clustering on the last 50 commits, so we can catch regression sooner.

BUG=skia:

Review URL: https://codereview.chromium.org/765423003
Adding SSL support + nginx config to skiamonitor

BUG=skia:3171

Review URL: https://codereview.chromium.org/764343005