Bump version in appveyor.yml
Bump version in appveyor.yml

Right now CI builds are coming through as 2.3.1-ci<build number>. Because of [version ordering](https://docs.microsoft.com/en-us/nuget/create-packages/prerelease-packages) this causes them to not appear as prerelease updates to the current 2.3.1 release.
ButchersBoy/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit,ButchersBoy/MaterialDesignInXamlToolkit