Tested that non preview Visual Studio versions work with the -prerelease flag. Changing the code to match corefx implementation.
Tested that non preview Visual Studio versions work with the -prerelease
flag. Changing the code to match corefx implementation.

MichaelSimons/core-setup,MichaelSimons/core-setup,MichaelSimons/core-setup,crummel/dotnet_core-setup,crummel/dotnet_core-setup,MichaelSimons/core-setup,crummel/dotnet_core-setup,MichaelSimons/core-setup,crummel/dotnet_core-setup,crummel/dotnet_core-setup,crummel/dotnet_core-setup,MichaelSimons/core-setup