Bump docserver to 3-48-0.

Also adjusts cron runtime to 180 minutes. It's been at 240 on the
live instance for a while but was never committed that way.

Refresh cycles are taking less time now, so dropping down to 180.

R=kalman@chromium.org
NOTRY=True
BUG=None

Review URL: https://codereview.chromium.org/977213002

Cr-Commit-Position: refs/heads/master@{#319156}

chrome/common/extensions/docs/server2/app.yaml

 application: chrome-apps-doc
-version: 3-47-4
+version: 3-48-0
 runtime: python27
 api_version: 1
 threadsafe: false

chrome/common/extensions/docs/server2/cron.yaml

 cron:
 - description: Repopulates all cached data.
   url: /_cron
-  schedule: every 60 minutes
-  target: 3-47-4
+  schedule: every 180 minutes
+  target: 3-48-0