fix fragile gradebook history code
fix fragile gradebook history code

in specs, graded_at.to_i may be the same between versions; just rely on the sorting from the db

Change-Id: I8385ff3a6096415b26ecabc9ff879f290a97f234
Reviewed-on: https://gerrit.instructure.com/56237
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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