PackageLicenseUrl is being deprecated

[luigiberrettini]

The licenseUrl metadata element of .nuspec files is being deprecated in favor of the license one

Looking at the NuGet/Home#4628 issue it's clear that project files must be edited replacing the PackageLicenseUrl tag with the PackageLicenseExpression one

Essentially <PackageLicenseUrl>https://github.com/luigiberrettini/NLog.Targets.Syslog/blob/master/LICENSE</PackageLicenseUrl> must be replaced by <PackageLicenseExpression>BSD-3-Clause</PackageLicenseExpression>