Raise NETStandard.Library version to 2.0.3

[slang25]

Relates to discussion in #398

With the sample app, the project.assets.json file (a sort of transient paket.lock for nuget) drops from 4830 lines vs 1300 lines.

A downside to this is:

• It's weird to have to do this
• We get a compiler warning in the Giraffe project that is hard to suppress

I'm not convinced we should merge this, I think the real solution is to press on with a 2.2.0 TaskBuilder.fs, or migrate to ply.

[dustinmoris]

Agree this feels like an unnecessary hack. TaskBuilder already has the change in an alpha release so we should just push for the proper release there or we could release a pre-release of Giraffe with the pre-release of TaskBuilder for those who can't wait.

[dustinmoris]

I think we should restrain from merging this.

[slang25]

Agreed 👍