Bump to Xamarin.Forms 4.7.0.968 (jonathanpeppers#59)

Fixes: jonathanpeppers#58

We can also drop the `-pre` suffix on this package.

Fix API breakage in Xamarin.Forms 4.7-pre (jonathanpeppers#55)

* Implemented `IPlatformServices.GetHash`
* Updated other various NuGet packages like NUnit

Co-authored-by: Jonathan Peppers <jonathan.peppers@gmail.com>

Update README.md

Bump version to 3.5.0.3

[build] locate MSBuild properly (jonathanpeppers#43)

Context: nunit/nunit#3194

I tried to build this repo on a new machine that only has VS 2019 installed. It didn't build...

I pulled a fix over that NUnit is using in their Cake script.

Let's just use MSBuild Restore target

Support for Xamarin.Forms 3.5 (jonathanpeppers#36)

Fixes: jonathanpeppers#34

Originally targeted a XF 4.0 preview, but we need to target 3.5 instead.

*also tweaked build.cake*

[build] prep for a prerelease targeting XF 4.0 (jonathanpeppers#33)

* [build] prep for a prerelease targeting XF 4.0

Let's also create build artifacts on AppVeyor of the nupkg files

[appveyor] we need to *package* the NuGet