Nuspec improvements

[fcastilloec]

• Removes deprecated summary option and use only description. See https://docs.microsoft.com/en-us/nuget/reference/nuspec#summary
• Removes deprecated iconUrl option and replace it with iconNuget. See https://docs.microsoft.com/en-us/nuget/reference/nuspec#iconurl
• Make spec.ejs easier to read by showing the required elements first and without if statements