On running \'pack\' I am getting the following error...
2017-07-05T10:37:09.7762333Z ##[section]Starting: NuGet pack 2017-07-05T10:37:09.7762333Z ===========
This seems to be an issue using the Nuget pack item built in to VSTS. I swapped it out for a Command Line element to call dotnet pack instead and it now works.