Docserver: Increment the server version.

This CL increments the Docserver version. I am testing auto-updating the server
for github hosted repositories and this should help guard against any
unintentional changes to the currently served documentation.

BUG=820605

Change-Id: I3f68767da92e5b120904fc777862e72cea4269de
Reviewed-on: https://chromium-review.googlesource.com/1123373Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572057}

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

 application: chrome-apps-doc
-version: 3-56-0
+version: 3-57-0
 runtime: python27
 api_version: 1
 threadsafe: false