bump rack to 2.0.7
bump rack to 2.0.7

https://github.com/rack/rack/compare/2.0.6...2.0.7

Change-Id: I89960a829d3dce7759750c67ac55341a85507ff7
Reviewed-on: https://gerrit.instructure.com/212272
Tested-by: Jenkins
Reviewed-by: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

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