Update IdentityModel dependencies to 5.1.2

[Tratcher]

@blowdart @muratg recommended for 1.1.1

[muratg]

Putting this in 1.1.1 for the patch release triage.

[blowdart]

Not just recommended. Mandatory.

[Tratcher]

https://technet.microsoft.com/library/security/3214296

[Eilon]

This patch bug is approved. Please use the normal code review process w/ a PR and make sure the fix is in the correct branch, then close the bug and mark it as done.