Frozen JitBench Version

[hoyosjs]

Given that JitBench keeps breaking, I've decided to freeze the version of dotnet and JitBench until the testing of fresh CoreCLR/CoreFX and ASP.NET releases becomes priority 0.

[hoyosjs]

Frozen to:

• Shared Framework: 2.1.0-preview2-25513-01
• SDK: 2.1.0-preview1-006776
• ASP.NET: 2.0.0-rtm-26320

Brittle as depends on local ASP.NET and packages often are deleted from the feed. Fix might rely on studying the scripts in jitbench as to how they determine the version of the frameworks in the cross-gen scripts.