Extensions Docserver: Bump the version.

This CL bumps the Docserver version to 3.55.0. We uploaded new templates to the
chrome webstore GCS bucket and this is required to see whether the update works.

BUG=796031

Change-Id: Ibfeb8e549c642390f47918bb6c0a8e1a1279577c
Reviewed-on: https://chromium-review.googlesource.com/957813Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542313}

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

 application: chrome-apps-doc
-version: 3-54-0
+version: 3-55-0
 runtime: python27
 api_version: 1
 threadsafe: false