Bump rack from 1.6.11 to 1.6.13

[dependabot[bot]]

Bumps rack from 1.6.11 to 1.6.13.

Commits

• 47a1fd7 bump version
• b8dc520 Handle case where session id key is requested but it is missing
• 698a060 Merge pull request #1462 from jeremyevans/sessionid-to_s
• de902e4 Merge branch '1-6-sec' into 1-6-stable
• b7d6546 Bump version
• d3e2f88 making diff smaller
• 99a8a87 fix memcache tests on 1.6
• f2cb48e fix tests on 1.6
• 7ff635c Introduce a new base class to avoid breaking when upgrading
• 3232f93 Add a version prefix to the private id to make easier to migrate old values
• Additional commits viewable in compare view

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