(#172) Removed artifact publishing from yml file

- This will instead be done within the psake build script
(#172) Added nuget package caching

- Now that the Pull Request issues in AppVeyor have been correct, we can add back in the nuget package caching, which should speed up the build somewhat.
(#172) Testing reading of Environment Variable

- Rather than use $env nomenclature, let's use GetEnvironmentVariable instead
- If this works, let's use _ instead of . in GitVersion