(#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) Altered how GitVersion is called

- When running in AppVeyor, run GitVersion with /output buildserver and read environment variable to get LegacySemVer version number
- When not running in AppVeyor, call GitVersion directly, and parse the JOSN output to get the LegacySemVer version number