Update to CoreCLR 2.0

[brettcannon]

Now that it's out, we need to target the latest released version of CoreCLR. We will need to decide if we want to target a specific branch or go by tags to handle updating our submodule details.

[brettcannon]

Oh, and properly document how the update to track upstream.

[brettcannon]

And @tonybaloney might have some ideas of how best to document and upgrade our submodule.

[tonybaloney]

Can do!

[emmatyping]

Would it be better to update to 2.0? Preview 2 should come out in the next few weeks.

EDIT: On the creators update, the new Windows SDK breaks building with VS 2015. If we update to Preview 2 of 2.0, we will get support for VS 2017. Right now I've patched VS build tools as seen here, but updating is a better longer term solution imo.

[brettcannon]

So @DinoV will know best, but he's not available right now so we will have to wait until he's available to comment.

[brettcannon]

@DinoV ping?

[brettcannon]

Since .NET Core 2.0 is now out there's no reason not to upgrade to it.

[tonybaloney]

Implemented in #237 (+3 versions)