update the example session_store.yml

Use encrypted_cookie_store by default, since it's the best option. This
fixes the session timeout specs failing with the default config.

Also remove the documentation on memcache/redis session store, since
those are deprecated.

refs CNVS-10467 (fixes a travis.ci build error)

Change-Id: Iafe633f601565d4306bb2f144d5d846d6fd527d5
Reviewed-on: https://gerrit.instructure.com/28946
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>